diff --git a/runtime/vm/object.cc b/runtime/vm/object.cc
index 771e6e8..c8c2e6a 100644
--- a/runtime/vm/object.cc
+++ b/runtime/vm/object.cc
@@ -2448,13 +2448,12 @@
 
       function ^= code.function();
       // If function uses dependent code switch it to unoptimized.
-      if (function.CurrentCode() == code.raw()) {
+      if (code.is_optimized() && (function.CurrentCode() == code.raw())) {
         ReportSwitchingCode(code);
-        if (code.is_optimized()) {
-          function.SwitchToUnoptimizedCode();
-        } else {
-          function.ClearCode();
-        }
+        function.SwitchToUnoptimizedCode();
+      } else if (function.unoptimized_code() == code.raw()) {
+        ReportSwitchingCode(code);
+        function.ClearCode();
       }
     }
   }
@@ -9235,8 +9234,9 @@
 
   virtual void ReportSwitchingCode(const Code& code) {
     if (FLAG_trace_deoptimization || FLAG_trace_deoptimization_verbose) {
-      OS::PrintErr("Prefix '%s': deleting code for function '%s'\n",
+      OS::PrintErr("Prefix '%s': deleting %s code for function '%s'\n",
         String::Handle(prefix_.name()).ToCString(),
+        code.is_optimized() ? "optimized" : "unoptimized",
         Function::Handle(code.function()).ToCString());
     }
   }
diff --git a/sdk/lib/_internal/compiler/implementation/types/dictionary_type_mask.dart b/sdk/lib/_internal/compiler/implementation/types/dictionary_type_mask.dart
index 72d73e2..3bd0f6f 100644
--- a/sdk/lib/_internal/compiler/implementation/types/dictionary_type_mask.dart
+++ b/sdk/lib/_internal/compiler/implementation/types/dictionary_type_mask.dart
@@ -90,7 +90,7 @@
         }
       });
       return new DictionaryTypeMask(newForwardTo, null, null,
-                                    keyType, valueType, mappings);
+                                    newKeyType, newValueType, mappings);
     } else if (other.isMap &&
                (other.keyType != null) &&
                (other.valueType != null)) {
diff --git a/tests/compiler/dart2js_extra/17856_test.dart b/tests/compiler/dart2js_extra/17856_test.dart
new file mode 100644
index 0000000..1e6fa63
--- /dev/null
+++ b/tests/compiler/dart2js_extra/17856_test.dart
@@ -0,0 +1,38 @@
+// Copyright (c) 2014, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+import "package:expect/expect.dart";
+
+// Regression test for Issue 17856.
+
+void main() {
+  var all = { "a": new A(), "b": new B() };
+
+  A a = all["a"];
+  a.load();
+}
+
+class A {
+  Loader _loader = new Loader();
+
+  load() => _loader.loadAll({
+    'a1': {}
+  });
+}
+
+class B {
+  Loader _loader = new Loader();
+
+  load() => _loader.loadAll({
+    'a2': new DateTime.now(),
+  });
+}
+
+class Loader {
+  loadAll(Map assets) {
+    for(String key in assets.keys) {
+      Expect.isTrue(assets[key] is Map);
+    }
+  }
+}
diff --git a/tools/VERSION b/tools/VERSION
index c121fd1..3db5927 100644
--- a/tools/VERSION
+++ b/tools/VERSION
@@ -28,4 +28,4 @@
 MINOR 3
 PATCH 0
 PRERELEASE 7
-PRERELEASE_PATCH 5
+PRERELEASE_PATCH 6