Add a new flag to dart2js: --trust-type-annotations and implement it in the types inferrer.

sdk/lib/_internal/compiler/implementation/compiler.dart

Index: sdk/lib/_internal/compiler/implementation/compiler.dart
===================================================================
--- sdk/lib/_internal/compiler/implementation/compiler.dart	(revision 22743)
+++ sdk/lib/_internal/compiler/implementation/compiler.dart	(working copy)
@@ -251,6 +251,7 @@
   final bool enableMinification;
   final bool enableTypeAssertions;
   final bool enableUserAssertions;
+  final bool trustTypeAnnotations;
   final bool enableConcreteTypeInference;
   /**
    * The maximum size of a concrete type before it widens to dynamic during
@@ -438,8 +439,9 @@
   bool hasCrashed = false;
 
   Compiler({this.tracer: const Tracer(),
-            this.enableTypeAssertions: false,
+            bool enableTypeAssertions: false,
             this.enableUserAssertions: false,
+            bool trustTypeAnnotations: false,
             this.enableConcreteTypeInference: false,
             this.maxConcreteTypeSize: 5,
             this.enableMinification: false,
@@ -460,6 +462,9 @@
             List<String> strips: const []})
       : this.analyzeOnly = analyzeOnly || analyzeSignaturesOnly,
         this.analyzeSignaturesOnly = analyzeSignaturesOnly,
+        this.enableTypeAssertions = enableTypeAssertions,
+        this.trustTypeAnnotations =

[kasperl, 2013/05/16 13:17:20]

Maybe keep the flags separate here, so the inferrer can use them separately if
it needs to?

[ngeoffray, 2013/05/17 14:34:25]

Done.

+            enableTypeAssertions || trustTypeAnnotations,
         this.outputProvider =
             (outputProvider == null) ? NullSink.outputProvider : outputProvider