Fix source maps strong mode runtime failure in dart2js.

Fix for https://github.com/dart-lang/sdk/issues/33332

Change-Id: I16c464b5dec85ccef55ecbe2e75d67dbf3481927
Reviewed-on: https://dart-review.googlesource.com/58480
Reviewed-by: Stephen Adams <sra@google.com>
Commit-Queue: Emily Fortuna <efortuna@google.com>
diff --git a/pkg/compiler/lib/src/io/source_information.dart b/pkg/compiler/lib/src/io/source_information.dart
index 5cbb473..59e9b2e 100644
--- a/pkg/compiler/lib/src/io/source_information.dart
+++ b/pkg/compiler/lib/src/io/source_information.dart
@@ -43,7 +43,7 @@
   /// Create a [SourceInformationBuilder] for [member].
   SourceInformationBuilder<T> createBuilderForContext(
       covariant MemberEntity member) {
-    return const SourceInformationBuilder();
+    return new SourceInformationBuilder<T>();
   }
 
   /// Generate [SourceInformation] marker for non-preamble code.