dart2js: Set inferred return types for async, async*, and sync* methods.

pkg/compiler/lib/src/types/types.dart

Index: pkg/compiler/lib/src/types/types.dart
diff --git a/pkg/compiler/lib/src/types/types.dart b/pkg/compiler/lib/src/types/types.dart
index 733d119f874ec197978646b2cd78667b0d08c08e..b8591aafc190d198021634e204178c1976c509cf 100644
--- a/pkg/compiler/lib/src/types/types.dart
+++ b/pkg/compiler/lib/src/types/types.dart
@@ -87,6 +87,9 @@ class TypesTask extends CompilerTask {
   TypeMask constMapTypeCache;
   TypeMask stringTypeCache;
   TypeMask typeTypeCache;
+  TypeMask syncStarIterableTypeCache;
+  TypeMask asyncFutureTypeCache;
+  TypeMask asyncStarStreamTypeCache;
 
   TypeMask get dynamicType {
     if (dynamicTypeCache == null) {
@@ -232,6 +235,30 @@ class TypesTask extends CompilerTask {
     return typeTypeCache;
   }
 
+  TypeMask get syncStarIterableType {
+    if (syncStarIterableTypeCache == null) {
+      syncStarIterableTypeCache = new TypeMask.nonNullExact(
+          compiler.backend.syncStarIterableImplementation, compiler.world);
+    }
+    return syncStarIterableTypeCache;
+  }
+
+  TypeMask get asyncFutureType {
+    if (asyncFutureTypeCache == null) {
+      asyncFutureTypeCache = new TypeMask.nonNullExact(
+          compiler.backend.asyncFutureImplementation, compiler.world);
+    }
+    return asyncFutureTypeCache;
+  }
+
+  TypeMask get asyncStarStreamType {
+    if (asyncStarStreamTypeCache == null) {
+      asyncStarStreamTypeCache = new TypeMask.nonNullExact(
+          compiler.backend.asyncStarStreamImplementation, compiler.world);
+    }
+    return asyncStarStreamTypeCache;
+  }
+
   TypeMask get nullType {
     if (nullTypeCache == null) {
       // TODO(johnniwinther): Assert that the null type has been resolved.