diff --git a/pkg/front_end/testcases/async_function.dart.direct.transformed.expect b/pkg/front_end/testcases/async_function.dart.direct.transformed.expect
index 6638f7c..d09ecec 100644
--- a/pkg/front_end/testcases/async_function.dart.direct.transformed.expect
+++ b/pkg/front_end/testcases/async_function.dart.direct.transformed.expect
@@ -2,7 +2,6 @@
 import self as self;
 import "dart:async" as asy;
 import "dart:core" as core;
-import "dart:_internal" as _in;
 
 static field core::List<core::String> stringList = <dynamic>["bar"];
 static method asyncString() → asy::Future<core::String> /* originally async */ {
@@ -119,7 +118,7 @@
           else
             [yield] null;
           [yield] let dynamic #t1 = asy::_awaitHelper(self::asyncString(), :async_op_then, :async_op_error, :async_op) in null;
-          if(:controller.{asy::_AsyncStarStreamController::add}(_in::unsafeCast<core::String>(:result)))
+          if(:controller.{asy::_AsyncStarStreamController::add}(:result))
             return null;
           else
             [yield] null;
@@ -188,7 +187,7 @@
       #L5:
       {
         [yield] let dynamic #t2 = asy::_awaitHelper(self::asyncString(), :async_op_then, :async_op_error, :async_op) in null;
-        core::String str = _in::unsafeCast<core::String>(:result);
+        core::String str = :result;
       }
       asy::_completeOnAsyncReturn(:async_completer, :return_value);
       return;
diff --git a/pkg/front_end/testcases/async_nested.dart.direct.transformed.expect b/pkg/front_end/testcases/async_nested.dart.direct.transformed.expect
index 1d8ba8f..da3a56f 100644
--- a/pkg/front_end/testcases/async_nested.dart.direct.transformed.expect
+++ b/pkg/front_end/testcases/async_nested.dart.direct.transformed.expect
@@ -2,7 +2,6 @@
 import self as self;
 import "dart:core" as core;
 import "dart:async" as asy;
-import "dart:_internal" as _in;
 
 class Node extends core::Object {
   final field core::List<self::Node> nested;
@@ -44,7 +43,7 @@
         [yield] let dynamic #t9 = asy::_awaitHelper(asy::Future::value<dynamic>(new self::Node::•("3", <dynamic>[:result])), :async_op_then, :async_op_error, :async_op) in null;
         :async_temporary_0 = :result;
         [yield] let dynamic #t10 = asy::_awaitHelper(asy::Future::value<dynamic>(new self::Node::•("10", <dynamic>[])), :async_op_then, :async_op_error, :async_op) in null;
-        self::Node node = new self::Node::•("1", <dynamic>[_in::unsafeCast<self::Node>(:async_temporary_2), :async_temporary_0, :result]);
+        self::Node node = new self::Node::•("1", <dynamic>[:async_temporary_2, :async_temporary_0, :result]);
         core::String actual = node.toSimpleString();
         core::print(actual);
         if(!actual.==(expected)) {
diff --git a/pkg/front_end/testcases/await.dart.direct.transformed.expect b/pkg/front_end/testcases/await.dart.direct.transformed.expect
index 655215a..7693f8c 100644
--- a/pkg/front_end/testcases/await.dart.direct.transformed.expect
+++ b/pkg/front_end/testcases/await.dart.direct.transformed.expect
@@ -2,7 +2,6 @@
 import self as self;
 import "dart:async" as asy;
 import "dart:core" as core;
-import "dart:_internal" as _in;
 
 static method main() → dynamic /* originally async */ {
   final asy::Completer<dynamic> :async_completer = asy::Completer::sync<dynamic>();
@@ -18,7 +17,7 @@
       #L1:
       {
         [yield] let dynamic #t1 = asy::_awaitHelper("Hello, World!", :async_op_then, :async_op_error, :async_op) in null;
-        core::print(_in::unsafeCast<core::String>(:result));
+        core::print(:result);
       }
       asy::_completeOnAsyncReturn(:async_completer, :return_value);
       return;
diff --git a/pkg/front_end/testcases/bug33206.dart.direct.transformed.expect b/pkg/front_end/testcases/bug33206.dart.direct.transformed.expect
index b27bff7..b5f9c88 100644
--- a/pkg/front_end/testcases/bug33206.dart.direct.transformed.expect
+++ b/pkg/front_end/testcases/bug33206.dart.direct.transformed.expect
@@ -2,7 +2,6 @@
 import self as self;
 import "dart:core" as core;
 import "dart:async" as asy;
-import "dart:_internal" as _in;
 
 class X extends core::Object {
   final field dynamic x;
@@ -90,10 +89,10 @@
       {
         final dynamic #t1 = new self::Y::•();
         [yield] let dynamic #t2 = asy::_awaitHelper(self::f1(), :async_op_then, :async_op_error, :async_op) in null;
-        final dynamic #t3 = #t1.f(_in::unsafeCast<core::List<core::Object>>(:result));
+        final dynamic #t3 = #t1.f(:result);
         final dynamic #t4 = #t1.f(self::f2());
         [yield] let dynamic #t5 = asy::_awaitHelper(self::f3(), :async_op_then, :async_op_error, :async_op) in null;
-        :return_value = new self::X::•(#t1, _in::unsafeCast<core::Object>(:result));
+        :return_value = new self::X::•(#t1, :result);
         break #L3;
       }
       asy::_completeOnAsyncReturn(:async_completer, :return_value);
@@ -122,7 +121,7 @@
       #L4:
       {
         [yield] let dynamic #t6 = asy::_awaitHelper(self::foo(), :async_op_then, :async_op_error, :async_op) in null;
-        core::print(_in::unsafeCast<self::X>(:result));
+        core::print(:result);
       }
       asy::_completeOnAsyncReturn(:async_completer, :return_value);
       return;
diff --git a/pkg/front_end/testcases/inference/async_await.dart.direct.transformed.expect b/pkg/front_end/testcases/inference/async_await.dart.direct.transformed.expect
index a358731..d797b5d 100644
--- a/pkg/front_end/testcases/inference/async_await.dart.direct.transformed.expect
+++ b/pkg/front_end/testcases/inference/async_await.dart.direct.transformed.expect
@@ -2,7 +2,6 @@
 import self as self;
 import "dart:core" as core;
 import "dart:async" as asy;
-import "dart:_internal" as _in;
 
 abstract class MyFuture extends core::Object implements asy::Future<core::int> {
   synthetic constructor •() → void
@@ -303,25 +302,25 @@
           return :async_completer.{asy::Completer::future};
         }
         [yield] let dynamic #t1 = asy::_awaitHelper(x0, :async_op_then, :async_op_error, :async_op) in null;
-        dynamic y0 = _in::unsafeCast<core::int>(:result);
+        dynamic y0 = :result;
         [yield] let dynamic #t2 = asy::_awaitHelper(x1, :async_op_then, :async_op_error, :async_op) in null;
-        dynamic y1 = _in::unsafeCast<core::int>(:result);
+        dynamic y1 = :result;
         [yield] let dynamic #t3 = asy::_awaitHelper(x2, :async_op_then, :async_op_error, :async_op) in null;
-        dynamic y2 = _in::unsafeCast<asy::Future<core::int>>(:result);
+        dynamic y2 = :result;
         [yield] let dynamic #t4 = asy::_awaitHelper(x3, :async_op_then, :async_op_error, :async_op) in null;
-        dynamic y3 = _in::unsafeCast<asy::FutureOr<core::int>>(:result);
+        dynamic y3 = :result;
         [yield] let dynamic #t5 = asy::_awaitHelper(x4, :async_op_then, :async_op_error, :async_op) in null;
-        dynamic y4 = _in::unsafeCast<self::MyFuture>(:result);
+        dynamic y4 = :result;
         [yield] let dynamic #t6 = asy::_awaitHelper(x5, :async_op_then, :async_op_error, :async_op) in null;
-        dynamic y5 = _in::unsafeCast<core::int>(:result);
+        dynamic y5 = :result;
         [yield] let dynamic #t7 = asy::_awaitHelper(x6, :async_op_then, :async_op_error, :async_op) in null;
-        dynamic y6 = _in::unsafeCast<asy::Future<core::int>>(:result);
+        dynamic y6 = :result;
         [yield] let dynamic #t8 = asy::_awaitHelper(x7, :async_op_then, :async_op_error, :async_op) in null;
-        dynamic y7 = _in::unsafeCast<asy::FutureOr<core::int>>(:result);
+        dynamic y7 = :result;
         [yield] let dynamic #t9 = asy::_awaitHelper(x8, :async_op_then, :async_op_error, :async_op) in null;
-        dynamic y8 = _in::unsafeCast<self::MyFuture>(:result);
+        dynamic y8 = :result;
         [yield] let dynamic #t10 = asy::_awaitHelper(x9, :async_op_then, :async_op_error, :async_op) in null;
-        dynamic y9 = _in::unsafeCast<core::int>(:result);
+        dynamic y9 = :result;
       }
       asy::_completeOnAsyncReturn(:async_completer, :return_value);
       return;
diff --git a/pkg/front_end/testcases/inference/downwards_inference_async_await.dart.direct.transformed.expect b/pkg/front_end/testcases/inference/downwards_inference_async_await.dart.direct.transformed.expect
index 8e76657..af02e87 100644
--- a/pkg/front_end/testcases/inference/downwards_inference_async_await.dart.direct.transformed.expect
+++ b/pkg/front_end/testcases/inference/downwards_inference_async_await.dart.direct.transformed.expect
@@ -2,7 +2,6 @@
 import self as self;
 import "dart:async" as asy;
 import "dart:core" as core;
-import "dart:_internal" as _in;
 
 static method main() → asy::Future<dynamic> /* originally async */ {
   final asy::Completer<dynamic> :async_completer = asy::Completer::sync<dynamic>();
@@ -19,7 +18,7 @@
       {
         dynamic d;
         [yield] let dynamic #t1 = asy::_awaitHelper(<dynamic>[d], :async_op_then, :async_op_error, :async_op) in null;
-        core::List<core::int> l0 = _in::unsafeCast<core::List<dynamic>>(:result);
+        core::List<core::int> l0 = :result;
         [yield] let dynamic #t2 = asy::_awaitHelper(asy::Future::value<dynamic>(<dynamic>[d]), :async_op_then, :async_op_error, :async_op) in null;
         core::List<core::int> l1 = :result;
       }
diff --git a/pkg/front_end/testcases/inference/downwards_inference_for_each.dart.direct.transformed.expect b/pkg/front_end/testcases/inference/downwards_inference_for_each.dart.direct.transformed.expect
index 8bbd33f..c6a2b38 100644
--- a/pkg/front_end/testcases/inference/downwards_inference_for_each.dart.direct.transformed.expect
+++ b/pkg/front_end/testcases/inference/downwards_inference_for_each.dart.direct.transformed.expect
@@ -2,7 +2,6 @@
 import self as self;
 import "dart:core" as core;
 import "dart:async" as asy;
-import "dart:_internal" as _in;
 
 abstract class MyStream<T extends core::Object = dynamic> extends asy::Stream<self::MyStream::T> {
   static factory •<T extends core::Object = dynamic>() → self::MyStream<self::MyStream::•::T>
@@ -50,7 +49,7 @@
             while (true) {
               dynamic #t3 = :product-mode ?{dynamic} null : asy::_asyncStarMoveNextHelper(:stream);
               [yield] let dynamic #t4 = asy::_awaitHelper(:for-iterator.{asy::_StreamIterator::moveNext}(), :async_op_then, :async_op_error, :async_op) in null;
-              if(_in::unsafeCast<core::bool>(:result)) {
+              if(:result) {
                 dynamic x = :for-iterator.{asy::_StreamIterator::current};
                 {}
               }
@@ -73,7 +72,7 @@
             while (true) {
               dynamic #t6 = :product-mode ?{dynamic} null : asy::_asyncStarMoveNextHelper(:stream);
               [yield] let dynamic #t7 = asy::_awaitHelper(:for-iterator.{asy::_StreamIterator::moveNext}(), :async_op_then, :async_op_error, :async_op) in null;
-              if(_in::unsafeCast<core::bool>(:result)) {
+              if(:result) {
                 dynamic x = :for-iterator.{asy::_StreamIterator::current};
                 {}
               }
@@ -96,7 +95,7 @@
             while (true) {
               dynamic #t9 = :product-mode ?{dynamic} null : asy::_asyncStarMoveNextHelper(:stream);
               [yield] let dynamic #t10 = asy::_awaitHelper(:for-iterator.{asy::_StreamIterator::moveNext}(), :async_op_then, :async_op_error, :async_op) in null;
-              if(_in::unsafeCast<core::bool>(:result)) {
+              if(:result) {
                 core::Object x = :for-iterator.{asy::_StreamIterator::current};
                 {}
               }
@@ -119,7 +118,7 @@
             while (true) {
               dynamic #t12 = :product-mode ?{dynamic} null : asy::_asyncStarMoveNextHelper(:stream);
               [yield] let dynamic #t13 = asy::_awaitHelper(:for-iterator.{asy::_StreamIterator::moveNext}(), :async_op_then, :async_op_error, :async_op) in null;
-              if(_in::unsafeCast<core::bool>(:result)) {
+              if(:result) {
                 final dynamic #t14 = :for-iterator.{asy::_StreamIterator::current};
                 {
                   d = #t14;
@@ -144,7 +143,7 @@
             while (true) {
               dynamic #t16 = :product-mode ?{dynamic} null : asy::_asyncStarMoveNextHelper(:stream);
               [yield] let dynamic #t17 = asy::_awaitHelper(:for-iterator.{asy::_StreamIterator::moveNext}(), :async_op_then, :async_op_error, :async_op) in null;
-              if(_in::unsafeCast<core::bool>(:result)) {
+              if(:result) {
                 final dynamic #t18 = :for-iterator.{asy::_StreamIterator::current};
                 {
                   o = #t18;
@@ -204,7 +203,7 @@
             while (true) {
               dynamic #t20 = :product-mode ?{dynamic} null : asy::_asyncStarMoveNextHelper(:stream);
               [yield] let dynamic #t21 = asy::_awaitHelper(:for-iterator.{asy::_StreamIterator::moveNext}(), :async_op_then, :async_op_error, :async_op) in null;
-              if(_in::unsafeCast<core::bool>(:result)) {
+              if(:result) {
                 core::int x = :for-iterator.{asy::_StreamIterator::current};
                 {}
               }
@@ -227,7 +226,7 @@
             while (true) {
               dynamic #t23 = :product-mode ?{dynamic} null : asy::_asyncStarMoveNextHelper(:stream);
               [yield] let dynamic #t24 = asy::_awaitHelper(:for-iterator.{asy::_StreamIterator::moveNext}(), :async_op_then, :async_op_error, :async_op) in null;
-              if(_in::unsafeCast<core::bool>(:result)) {
+              if(:result) {
                 dynamic x = :for-iterator.{asy::_StreamIterator::current};
                 {}
               }
diff --git a/pkg/front_end/testcases/inference/for_each_downcast_iterable.dart.direct.transformed.expect b/pkg/front_end/testcases/inference/for_each_downcast_iterable.dart.direct.transformed.expect
index 8ac230e..46d4a06 100644
--- a/pkg/front_end/testcases/inference/for_each_downcast_iterable.dart.direct.transformed.expect
+++ b/pkg/front_end/testcases/inference/for_each_downcast_iterable.dart.direct.transformed.expect
@@ -2,7 +2,6 @@
 import self as self;
 import "dart:async" as asy;
 import "dart:core" as core;
-import "dart:_internal" as _in;
 
 static method test() → dynamic /* originally async */ {
   final asy::Completer<dynamic> :async_completer = asy::Completer::sync<dynamic>();
@@ -33,7 +32,7 @@
             while (true) {
               dynamic #t1 = :product-mode ?{dynamic} null : asy::_asyncStarMoveNextHelper(:stream);
               [yield] let dynamic #t2 = asy::_awaitHelper(:for-iterator.{asy::_StreamIterator::moveNext}(), :async_op_then, :async_op_error, :async_op) in null;
-              if(_in::unsafeCast<core::bool>(:result)) {
+              if(:result) {
                 dynamic x = :for-iterator.{asy::_StreamIterator::current};
                 {}
               }
@@ -60,7 +59,7 @@
             while (true) {
               dynamic #t5 = :product-mode ?{dynamic} null : asy::_asyncStarMoveNextHelper(:stream);
               [yield] let dynamic #t6 = asy::_awaitHelper(:for-iterator.{asy::_StreamIterator::moveNext}(), :async_op_then, :async_op_error, :async_op) in null;
-              if(_in::unsafeCast<core::bool>(:result)) {
+              if(:result) {
                 final dynamic #t7 = :for-iterator.{asy::_StreamIterator::current};
                 {
                   y = #t7;
diff --git a/pkg/front_end/testcases/inference/future_then.dart.direct.transformed.expect b/pkg/front_end/testcases/inference/future_then.dart.direct.transformed.expect
index 6159047..f27ed38 100644
--- a/pkg/front_end/testcases/inference/future_then.dart.direct.transformed.expect
+++ b/pkg/front_end/testcases/inference/future_then.dart.direct.transformed.expect
@@ -2,7 +2,6 @@
 import self as self;
 import "dart:core" as core;
 import "dart:async" as asy;
-import "dart:_internal" as _in;
 
 class MyFuture<T extends core::Object = dynamic> extends core::Object implements asy::Future<self::MyFuture::T> {
   constructor •() → void
@@ -38,7 +37,7 @@
         #L1:
         {
           [yield] let dynamic #t1 = asy::_awaitHelper(asy::Future::value<core::int>(3), :async_op_then, :async_op_error, :async_op) in null;
-          :return_value = _in::unsafeCast<core::int>(:result);
+          :return_value = :result;
           break #L1;
         }
         asy::_completeOnAsyncReturn(:async_completer, :return_value);
@@ -67,7 +66,7 @@
         #L2:
         {
           [yield] let dynamic #t2 = asy::_awaitHelper(asy::Future::value<core::int>(3), :async_op_then, :async_op_error, :async_op) in null;
-          :return_value = _in::unsafeCast<core::int>(:result);
+          :return_value = :result;
           break #L2;
         }
         asy::_completeOnAsyncReturn(:async_completer, :return_value);
diff --git a/pkg/front_end/testcases/inference/future_then_2.dart.direct.transformed.expect b/pkg/front_end/testcases/inference/future_then_2.dart.direct.transformed.expect
index d395a35..20ee8d1 100644
--- a/pkg/front_end/testcases/inference/future_then_2.dart.direct.transformed.expect
+++ b/pkg/front_end/testcases/inference/future_then_2.dart.direct.transformed.expect
@@ -2,7 +2,6 @@
 import self as self;
 import "dart:core" as core;
 import "dart:async" as asy;
-import "dart:_internal" as _in;
 
 class MyFuture<T extends core::Object = dynamic> extends core::Object implements asy::Future<self::MyFuture::T> {
   constructor •() → void
@@ -38,7 +37,7 @@
         #L1:
         {
           [yield] let dynamic #t1 = asy::_awaitHelper(new self::MyFuture::value<core::int>(3), :async_op_then, :async_op_error, :async_op) in null;
-          :return_value = _in::unsafeCast<core::int>(:result);
+          :return_value = :result;
           break #L1;
         }
         asy::_completeOnAsyncReturn(:async_completer, :return_value);
@@ -67,7 +66,7 @@
         #L2:
         {
           [yield] let dynamic #t2 = asy::_awaitHelper(new self::MyFuture::value<core::int>(3), :async_op_then, :async_op_error, :async_op) in null;
-          :return_value = _in::unsafeCast<core::int>(:result);
+          :return_value = :result;
           break #L2;
         }
         asy::_completeOnAsyncReturn(:async_completer, :return_value);
diff --git a/pkg/front_end/testcases/inference/future_then_3.dart.direct.transformed.expect b/pkg/front_end/testcases/inference/future_then_3.dart.direct.transformed.expect
index 9131ddc..c6d3766 100644
--- a/pkg/front_end/testcases/inference/future_then_3.dart.direct.transformed.expect
+++ b/pkg/front_end/testcases/inference/future_then_3.dart.direct.transformed.expect
@@ -2,7 +2,6 @@
 import self as self;
 import "dart:core" as core;
 import "dart:async" as asy;
-import "dart:_internal" as _in;
 
 class MyFuture<T extends core::Object = dynamic> extends core::Object implements asy::Future<self::MyFuture::T> {
   constructor •() → void
@@ -38,7 +37,7 @@
         #L1:
         {
           [yield] let dynamic #t1 = asy::_awaitHelper(asy::Future::value<core::int>(3), :async_op_then, :async_op_error, :async_op) in null;
-          :return_value = _in::unsafeCast<core::int>(:result);
+          :return_value = :result;
           break #L1;
         }
         asy::_completeOnAsyncReturn(:async_completer, :return_value);
@@ -67,7 +66,7 @@
         #L2:
         {
           [yield] let dynamic #t2 = asy::_awaitHelper(asy::Future::value<core::int>(3), :async_op_then, :async_op_error, :async_op) in null;
-          :return_value = _in::unsafeCast<core::int>(:result);
+          :return_value = :result;
           break #L2;
         }
         asy::_completeOnAsyncReturn(:async_completer, :return_value);
diff --git a/pkg/front_end/testcases/inference/future_then_4.dart.direct.transformed.expect b/pkg/front_end/testcases/inference/future_then_4.dart.direct.transformed.expect
index 9fc9fc2..8f4e8ca 100644
--- a/pkg/front_end/testcases/inference/future_then_4.dart.direct.transformed.expect
+++ b/pkg/front_end/testcases/inference/future_then_4.dart.direct.transformed.expect
@@ -2,7 +2,6 @@
 import self as self;
 import "dart:core" as core;
 import "dart:async" as asy;
-import "dart:_internal" as _in;
 
 class MyFuture<T extends core::Object = dynamic> extends core::Object implements asy::Future<self::MyFuture::T> {
   constructor •() → void
@@ -38,7 +37,7 @@
         #L1:
         {
           [yield] let dynamic #t1 = asy::_awaitHelper(new self::MyFuture::value<core::int>(3), :async_op_then, :async_op_error, :async_op) in null;
-          :return_value = _in::unsafeCast<core::int>(:result);
+          :return_value = :result;
           break #L1;
         }
         asy::_completeOnAsyncReturn(:async_completer, :return_value);
@@ -67,7 +66,7 @@
         #L2:
         {
           [yield] let dynamic #t2 = asy::_awaitHelper(new self::MyFuture::value<core::int>(3), :async_op_then, :async_op_error, :async_op) in null;
-          :return_value = _in::unsafeCast<core::int>(:result);
+          :return_value = :result;
           break #L2;
         }
         asy::_completeOnAsyncReturn(:async_completer, :return_value);
diff --git a/pkg/front_end/testcases/inference/future_then_5.dart.direct.transformed.expect b/pkg/front_end/testcases/inference/future_then_5.dart.direct.transformed.expect
index c6b8288..a617678 100644
--- a/pkg/front_end/testcases/inference/future_then_5.dart.direct.transformed.expect
+++ b/pkg/front_end/testcases/inference/future_then_5.dart.direct.transformed.expect
@@ -2,7 +2,6 @@
 import self as self;
 import "dart:core" as core;
 import "dart:async" as asy;
-import "dart:_internal" as _in;
 
 class MyFuture<T extends core::Object = dynamic> extends core::Object implements asy::Future<self::MyFuture::T> {
   constructor •() → void
@@ -38,7 +37,7 @@
         #L1:
         {
           [yield] let dynamic #t1 = asy::_awaitHelper(new self::MyFuture::value<core::int>(3), :async_op_then, :async_op_error, :async_op) in null;
-          :return_value = _in::unsafeCast<core::int>(:result);
+          :return_value = :result;
           break #L1;
         }
         asy::_completeOnAsyncReturn(:async_completer, :return_value);
@@ -67,7 +66,7 @@
         #L2:
         {
           [yield] let dynamic #t2 = asy::_awaitHelper(new self::MyFuture::value<core::int>(3), :async_op_then, :async_op_error, :async_op) in null;
-          :return_value = _in::unsafeCast<core::int>(:result);
+          :return_value = :result;
           break #L2;
         }
         asy::_completeOnAsyncReturn(:async_completer, :return_value);
diff --git a/pkg/front_end/testcases/inference/future_then_6.dart.direct.transformed.expect b/pkg/front_end/testcases/inference/future_then_6.dart.direct.transformed.expect
index 8a90334..925090f 100644
--- a/pkg/front_end/testcases/inference/future_then_6.dart.direct.transformed.expect
+++ b/pkg/front_end/testcases/inference/future_then_6.dart.direct.transformed.expect
@@ -2,7 +2,6 @@
 import self as self;
 import "dart:core" as core;
 import "dart:async" as asy;
-import "dart:_internal" as _in;
 
 class MyFuture<T extends core::Object = dynamic> extends core::Object implements asy::Future<self::MyFuture::T> {
   constructor •() → void
@@ -38,7 +37,7 @@
         #L1:
         {
           [yield] let dynamic #t1 = asy::_awaitHelper(asy::Future::value<core::int>(3), :async_op_then, :async_op_error, :async_op) in null;
-          :return_value = _in::unsafeCast<core::int>(:result);
+          :return_value = :result;
           break #L1;
         }
         asy::_completeOnAsyncReturn(:async_completer, :return_value);
@@ -67,7 +66,7 @@
         #L2:
         {
           [yield] let dynamic #t2 = asy::_awaitHelper(asy::Future::value<core::int>(3), :async_op_then, :async_op_error, :async_op) in null;
-          :return_value = _in::unsafeCast<core::int>(:result);
+          :return_value = :result;
           break #L2;
         }
         asy::_completeOnAsyncReturn(:async_completer, :return_value);
diff --git a/pkg/front_end/testcases/inference/future_union_downwards_generic_method_with_future_return.dart.direct.transformed.expect b/pkg/front_end/testcases/inference/future_union_downwards_generic_method_with_future_return.dart.direct.transformed.expect
index 4461a83..89b8af9 100644
--- a/pkg/front_end/testcases/inference/future_union_downwards_generic_method_with_future_return.dart.direct.transformed.expect
+++ b/pkg/front_end/testcases/inference/future_union_downwards_generic_method_with_future_return.dart.direct.transformed.expect
@@ -2,7 +2,6 @@
 import self as self;
 import "dart:core" as core;
 import "dart:async" as asy;
-import "dart:_internal" as _in;
 
 class A extends core::Object {
   synthetic constructor •() → void
@@ -25,7 +24,7 @@
         asy::Future<core::List<self::A>> f1 = null;
         asy::Future<core::List<self::A>> f2 = null;
         [yield] let dynamic #t1 = asy::_awaitHelper(asy::Future::wait<dynamic>(<dynamic>[f1, f2]), :async_op_then, :async_op_error, :async_op) in null;
-        core::List<core::List<self::A>> merged = _in::unsafeCast<core::List<dynamic>>(:result);
+        core::List<core::List<self::A>> merged = :result;
       }
       asy::_completeOnAsyncReturn(:async_completer, :return_value);
       return;
diff --git a/pkg/front_end/testcases/inference/future_union_upwards_generic_methods.dart.direct.transformed.expect b/pkg/front_end/testcases/inference/future_union_upwards_generic_methods.dart.direct.transformed.expect
index ebe93d9..7bc149c 100644
--- a/pkg/front_end/testcases/inference/future_union_upwards_generic_methods.dart.direct.transformed.expect
+++ b/pkg/front_end/testcases/inference/future_union_upwards_generic_methods.dart.direct.transformed.expect
@@ -2,7 +2,6 @@
 import self as self;
 import "dart:core" as core;
 import "dart:async" as asy;
-import "dart:_internal" as _in;
 
 class A extends core::Object {
   synthetic constructor •() → void
@@ -36,9 +35,9 @@
         dynamic c = asy::Future::value<self::C>(new self::C::•());
         dynamic lll = <dynamic>[b, c];
         [yield] let dynamic #t1 = asy::_awaitHelper(asy::Future::wait<dynamic>(lll), :async_op_then, :async_op_error, :async_op) in null;
-        dynamic result = _in::unsafeCast<core::List<dynamic>>(:result);
+        dynamic result = :result;
         [yield] let dynamic #t2 = asy::_awaitHelper(asy::Future::wait<dynamic>(<dynamic>[b, c]), :async_op_then, :async_op_error, :async_op) in null;
-        dynamic result2 = _in::unsafeCast<core::List<dynamic>>(:result);
+        dynamic result2 = :result;
         core::List<self::A> list = result;
         list = result2;
       }
diff --git a/pkg/front_end/testcases/inference/infer_types_on_loop_indices_for_each_loop_async.dart.direct.transformed.expect b/pkg/front_end/testcases/inference/infer_types_on_loop_indices_for_each_loop_async.dart.direct.transformed.expect
index dfe5531..8c8c294 100644
--- a/pkg/front_end/testcases/inference/infer_types_on_loop_indices_for_each_loop_async.dart.direct.transformed.expect
+++ b/pkg/front_end/testcases/inference/infer_types_on_loop_indices_for_each_loop_async.dart.direct.transformed.expect
@@ -2,7 +2,6 @@
 import self as self;
 import "dart:core" as core;
 import "dart:async" as asy;
-import "dart:_internal" as _in;
 
 class Foo extends core::Object {
   field core::int bar = 42;
@@ -40,7 +39,7 @@
               while (true) {
                 dynamic #t1 = :product-mode ?{dynamic} null : asy::_asyncStarMoveNextHelper(:stream);
                 [yield] let dynamic #t2 = asy::_awaitHelper(:for-iterator.{asy::_StreamIterator::moveNext}(), :async_op_then, :async_op_error, :async_op) in null;
-                if(_in::unsafeCast<core::bool>(:result)) {
+                if(:result) {
                   dynamic i = :for-iterator.{asy::_StreamIterator::current};
                   {
                     core::int x = i;
@@ -99,7 +98,7 @@
               while (true) {
                 dynamic #t4 = :product-mode ?{dynamic} null : asy::_asyncStarMoveNextHelper(:stream);
                 [yield] let dynamic #t5 = asy::_awaitHelper(:for-iterator.{asy::_StreamIterator::moveNext}(), :async_op_then, :async_op_error, :async_op) in null;
-                if(_in::unsafeCast<core::bool>(:result)) {
+                if(:result) {
                   dynamic i = :for-iterator.{asy::_StreamIterator::current};
                   {
                     core::int x = i;
@@ -160,7 +159,7 @@
             while (true) {
               dynamic #t7 = :product-mode ?{dynamic} null : asy::_asyncStarMoveNextHelper(:stream);
               [yield] let dynamic #t8 = asy::_awaitHelper(:for-iterator.{asy::_StreamIterator::moveNext}(), :async_op_then, :async_op_error, :async_op) in null;
-              if(_in::unsafeCast<core::bool>(:result)) {
+              if(:result) {
                 dynamic x = :for-iterator.{asy::_StreamIterator::current};
                 {
                   core::String y = x;
@@ -185,7 +184,7 @@
             while (true) {
               dynamic #t10 = :product-mode ?{dynamic} null : asy::_asyncStarMoveNextHelper(:stream);
               [yield] let dynamic #t11 = asy::_awaitHelper(:for-iterator.{asy::_StreamIterator::moveNext}(), :async_op_then, :async_op_error, :async_op) in null;
-              if(_in::unsafeCast<core::bool>(:result)) {
+              if(:result) {
                 dynamic x = :for-iterator.{asy::_StreamIterator::current};
                 {
                   core::String y = x;
@@ -210,7 +209,7 @@
             while (true) {
               dynamic #t13 = :product-mode ?{dynamic} null : asy::_asyncStarMoveNextHelper(:stream);
               [yield] let dynamic #t14 = asy::_awaitHelper(:for-iterator.{asy::_StreamIterator::moveNext}(), :async_op_then, :async_op_error, :async_op) in null;
-              if(_in::unsafeCast<core::bool>(:result)) {
+              if(:result) {
                 core::String x = :for-iterator.{asy::_StreamIterator::current};
                 {
                   core::String y = x;
@@ -236,7 +235,7 @@
             while (true) {
               dynamic #t16 = :product-mode ?{dynamic} null : asy::_asyncStarMoveNextHelper(:stream);
               [yield] let dynamic #t17 = asy::_awaitHelper(:for-iterator.{asy::_StreamIterator::moveNext}(), :async_op_then, :async_op_error, :async_op) in null;
-              if(_in::unsafeCast<core::bool>(:result)) {
+              if(:result) {
                 final dynamic #t18 = :for-iterator.{asy::_StreamIterator::current};
                 {
                   z = #t18;
@@ -263,7 +262,7 @@
             while (true) {
               dynamic #t20 = :product-mode ?{dynamic} null : asy::_asyncStarMoveNextHelper(:stream);
               [yield] let dynamic #t21 = asy::_awaitHelper(:for-iterator.{asy::_StreamIterator::moveNext}(), :async_op_then, :async_op_error, :async_op) in null;
-              if(_in::unsafeCast<core::bool>(:result)) {
+              if(:result) {
                 self::Foo x = :for-iterator.{asy::_StreamIterator::current};
                 {
                   dynamic y = x;
@@ -289,7 +288,7 @@
             while (true) {
               dynamic #t23 = :product-mode ?{dynamic} null : asy::_asyncStarMoveNextHelper(:stream);
               [yield] let dynamic #t24 = asy::_awaitHelper(:for-iterator.{asy::_StreamIterator::moveNext}(), :async_op_then, :async_op_error, :async_op) in null;
-              if(_in::unsafeCast<core::bool>(:result)) {
+              if(:result) {
                 self::Foo x = :for-iterator.{asy::_StreamIterator::current};
                 {
                   dynamic y = x;
@@ -315,7 +314,7 @@
             while (true) {
               dynamic #t26 = :product-mode ?{dynamic} null : asy::_asyncStarMoveNextHelper(:stream);
               [yield] let dynamic #t27 = asy::_awaitHelper(:for-iterator.{asy::_StreamIterator::moveNext}(), :async_op_then, :async_op_error, :async_op) in null;
-              if(_in::unsafeCast<core::bool>(:result)) {
+              if(:result) {
                 dynamic x = :for-iterator.{asy::_StreamIterator::current};
                 {
                   core::String y = x;
diff --git a/pkg/front_end/testcases/inference_new/for_each_invalid_iterable.dart.direct.transformed.expect b/pkg/front_end/testcases/inference_new/for_each_invalid_iterable.dart.direct.transformed.expect
index 547f641..3eefc91 100644
--- a/pkg/front_end/testcases/inference_new/for_each_invalid_iterable.dart.direct.transformed.expect
+++ b/pkg/front_end/testcases/inference_new/for_each_invalid_iterable.dart.direct.transformed.expect
@@ -2,7 +2,6 @@
 import self as self;
 import "dart:async" as asy;
 import "dart:core" as core;
-import "dart:_internal" as _in;
 
 static method test() → dynamic /* originally async */ {
   final asy::Completer<dynamic> :async_completer = asy::Completer::sync<dynamic>();
@@ -33,7 +32,7 @@
             while (true) {
               dynamic #t1 = :product-mode ?{dynamic} null : asy::_asyncStarMoveNextHelper(:stream);
               [yield] let dynamic #t2 = asy::_awaitHelper(:for-iterator.{asy::_StreamIterator::moveNext}(), :async_op_then, :async_op_error, :async_op) in null;
-              if(_in::unsafeCast<core::bool>(:result)) {
+              if(:result) {
                 core::int x = :for-iterator.{asy::_StreamIterator::current};
                 {}
               }
@@ -60,7 +59,7 @@
             while (true) {
               dynamic #t5 = :product-mode ?{dynamic} null : asy::_asyncStarMoveNextHelper(:stream);
               [yield] let dynamic #t6 = asy::_awaitHelper(:for-iterator.{asy::_StreamIterator::moveNext}(), :async_op_then, :async_op_error, :async_op) in null;
-              if(_in::unsafeCast<core::bool>(:result)) {
+              if(:result) {
                 final dynamic #t7 = :for-iterator.{asy::_StreamIterator::current};
                 {
                   y = #t7;
diff --git a/pkg/front_end/testcases/inference_new/for_each_outer_var_type.dart.direct.transformed.expect b/pkg/front_end/testcases/inference_new/for_each_outer_var_type.dart.direct.transformed.expect
index c914a79..1c7200b 100644
--- a/pkg/front_end/testcases/inference_new/for_each_outer_var_type.dart.direct.transformed.expect
+++ b/pkg/front_end/testcases/inference_new/for_each_outer_var_type.dart.direct.transformed.expect
@@ -2,7 +2,6 @@
 import self as self;
 import "dart:core" as core;
 import "dart:async" as asy;
-import "dart:_internal" as _in;
 
 class A extends core::Object {
   synthetic constructor •() → void
@@ -50,7 +49,7 @@
             while (true) {
               dynamic #t2 = :product-mode ?{dynamic} null : asy::_asyncStarMoveNextHelper(:stream);
               [yield] let dynamic #t3 = asy::_awaitHelper(:for-iterator.{asy::_StreamIterator::moveNext}(), :async_op_then, :async_op_error, :async_op) in null;
-              if(_in::unsafeCast<core::bool>(:result)) {
+              if(:result) {
                 final self::A #t4 = :for-iterator.{asy::_StreamIterator::current};
                 {
                   a = #t4;
@@ -78,7 +77,7 @@
             while (true) {
               dynamic #t7 = :product-mode ?{dynamic} null : asy::_asyncStarMoveNextHelper(:stream);
               [yield] let dynamic #t8 = asy::_awaitHelper(:for-iterator.{asy::_StreamIterator::moveNext}(), :async_op_then, :async_op_error, :async_op) in null;
-              if(_in::unsafeCast<core::bool>(:result)) {
+              if(:result) {
                 final self::A #t9 = :for-iterator.{asy::_StreamIterator::current};
                 {
                   b = #t9;
@@ -106,7 +105,7 @@
             while (true) {
               dynamic #t12 = :product-mode ?{dynamic} null : asy::_asyncStarMoveNextHelper(:stream);
               [yield] let dynamic #t13 = asy::_awaitHelper(:for-iterator.{asy::_StreamIterator::moveNext}(), :async_op_then, :async_op_error, :async_op) in null;
-              if(_in::unsafeCast<core::bool>(:result)) {
+              if(:result) {
                 final self::A #t14 = :for-iterator.{asy::_StreamIterator::current};
                 {
                   i = #t14;
@@ -134,7 +133,7 @@
             while (true) {
               dynamic #t17 = :product-mode ?{dynamic} null : asy::_asyncStarMoveNextHelper(:stream);
               [yield] let dynamic #t18 = asy::_awaitHelper(:for-iterator.{asy::_StreamIterator::moveNext}(), :async_op_then, :async_op_error, :async_op) in null;
-              if(_in::unsafeCast<core::bool>(:result)) {
+              if(:result) {
                 final dynamic #t19 = :for-iterator.{asy::_StreamIterator::current};
                 {
                   a = #t19;
diff --git a/pkg/kernel/bin/transform.dart b/pkg/kernel/bin/transform.dart
index e3db3b5..d04dda3 100755
--- a/pkg/kernel/bin/transform.dart
+++ b/pkg/kernel/bin/transform.dart
@@ -86,8 +86,8 @@
   final hierarchy = new ClassHierarchy(component);
   switch (options['transformation']) {
     case 'continuation':
-      component =
-          cont.transformComponent(coreTypes, hierarchy, component, syncAsync);
+      component = cont.transformComponent(
+          coreTypes, hierarchy, component, syncAsync, true);
       break;
     case 'resolve-mixins':
       mix.transformLibraries(
diff --git a/pkg/kernel/lib/transformations/async.dart b/pkg/kernel/lib/transformations/async.dart
index 4050e07..0d29e7e 100644
--- a/pkg/kernel/lib/transformations/async.dart
+++ b/pkg/kernel/lib/transformations/async.dart
@@ -115,7 +115,12 @@
   // expression [to] would have the same static type by wrapping it into
   // an appropriate unsafeCast.
   Expression assignType(Expression to, Expression from) {
-    final fromType = from.getStaticType(continuationRewriter.helper.env);
+    final env = continuationRewriter.helper.env;
+    if (!env.strongMode) {
+      return to;
+    }
+
+    final fromType = from.getStaticType(env);
     if (fromType == null || fromType == const DynamicType()) {
       return to;
     }
diff --git a/pkg/kernel/lib/transformations/continuation.dart b/pkg/kernel/lib/transformations/continuation.dart
index fd06825..56c03a9 100644
--- a/pkg/kernel/lib/transformations/continuation.dart
+++ b/pkg/kernel/lib/transformations/continuation.dart
@@ -26,8 +26,8 @@
 }
 
 void transformLibraries(CoreTypes coreTypes, ClassHierarchy hierarchy,
-    List<Library> libraries, bool syncAsync) {
-  var helper = new HelperNodes.fromCoreTypes(coreTypes, hierarchy);
+    List<Library> libraries, bool syncAsync, bool strongMode) {
+  var helper = new HelperNodes.fromCoreTypes(coreTypes, hierarchy, strongMode);
   var rewriter = new RecursiveContinuationRewriter(helper, syncAsync);
   for (var library in libraries) {
     rewriter.rewriteLibrary(library);
@@ -35,15 +35,15 @@
 }
 
 Component transformComponent(CoreTypes coreTypes, ClassHierarchy hierarchy,
-    Component component, bool syncAsync) {
-  var helper = new HelperNodes.fromCoreTypes(coreTypes, hierarchy);
+    Component component, bool syncAsync, bool strongMode) {
+  var helper = new HelperNodes.fromCoreTypes(coreTypes, hierarchy, strongMode);
   var rewriter = new RecursiveContinuationRewriter(helper, syncAsync);
   return rewriter.rewriteComponent(component);
 }
 
 Procedure transformProcedure(CoreTypes coreTypes, ClassHierarchy hierarchy,
-    Procedure procedure, bool syncAsync) {
-  var helper = new HelperNodes.fromCoreTypes(coreTypes, hierarchy);
+    Procedure procedure, bool syncAsync, bool strongMode) {
+  var helper = new HelperNodes.fromCoreTypes(coreTypes, hierarchy, strongMode);
   var rewriter = new RecursiveContinuationRewriter(helper, syncAsync);
   return rewriter.visitProcedure(procedure);
 }
@@ -1249,7 +1249,7 @@
       this.env);
 
   factory HelperNodes.fromCoreTypes(
-      CoreTypes coreTypes, ClassHierarchy hierarchy) {
+      CoreTypes coreTypes, ClassHierarchy hierarchy, bool strongMode) {
     return new HelperNodes._(
         coreTypes.asyncErrorWrapperHelperProcedure,
         coreTypes.asyncLibrary,
@@ -1294,6 +1294,6 @@
         coreTypes.boolClass,
         coreTypes.boolFromEnvironment,
         coreTypes.unsafeCast,
-        new TypeEnvironment(coreTypes, hierarchy, strongMode: true));
+        new TypeEnvironment(coreTypes, hierarchy, strongMode: strongMode));
   }
 }
diff --git a/pkg/vm/lib/target/vm.dart b/pkg/vm/lib/target/vm.dart
index f67260e..71b799a 100644
--- a/pkg/vm/lib/target/vm.dart
+++ b/pkg/vm/lib/target/vm.dart
@@ -67,7 +67,7 @@
 
     // TODO(kmillikin): Make this run on a per-method basis.
     transformAsync.transformLibraries(
-        coreTypes, hierarchy, libraries, flags.syncAsync);
+        coreTypes, hierarchy, libraries, flags.syncAsync, strongMode);
     logger?.call("Transformed async methods");
 
     callSiteAnnotator.transformLibraries(
@@ -80,7 +80,7 @@
       CoreTypes coreTypes, ClassHierarchy hierarchy, Procedure procedure,
       {void logger(String msg)}) {
     transformAsync.transformProcedure(
-        coreTypes, hierarchy, procedure, flags.syncAsync);
+        coreTypes, hierarchy, procedure, flags.syncAsync, strongMode);
     logger?.call("Transformed async functions");
   }