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

pkg/compiler/lib/src/inferrer/inferrer_visitor.dart

Index: pkg/compiler/lib/src/inferrer/inferrer_visitor.dart
diff --git a/pkg/compiler/lib/src/inferrer/inferrer_visitor.dart b/pkg/compiler/lib/src/inferrer/inferrer_visitor.dart
index 270ae43dfa8ced677114da19257c63eb60c9125a..ab2f50c5ac1901392cb070919a5aa7f23ee46e4f 100644
--- a/pkg/compiler/lib/src/inferrer/inferrer_visitor.dart
+++ b/pkg/compiler/lib/src/inferrer/inferrer_visitor.dart
@@ -55,6 +55,9 @@ abstract class TypeSystem<T> {
   T get constMapType;
   T get stringType;
   T get typeType;
+  T get syncStarIterableType;
+  T get asyncFutureType; // Subtype of Future returned by async methods.
+  T get asyncStarStreamType;
 
   T stringLiteralType(DartString value);
   T boolLiteralType(LiteralBool value);