Implement type inference for await expressions in front_end.

pkg/front_end/lib/src/fasta/type_inference/type_schema_environment.dart

Index: pkg/front_end/lib/src/fasta/type_inference/type_schema_environment.dart
diff --git a/pkg/front_end/lib/src/fasta/type_inference/type_schema_environment.dart b/pkg/front_end/lib/src/fasta/type_inference/type_schema_environment.dart
index 16beabb7bf8e4b37d3da6afa9aebed4077fb3836..1c1b7a20dacb1d5a76a885ed0afa45fc46347903 100644
--- a/pkg/front_end/lib/src/fasta/type_inference/type_schema_environment.dart
+++ b/pkg/front_end/lib/src/fasta/type_inference/type_schema_environment.dart
@@ -36,13 +36,13 @@ class TypeConstraint {
 }
 
 class TypeSchemaEnvironment extends TypeEnvironment {
+  @override
+  final bool strongMode;
+
   TypeSchemaEnvironment(
       CoreTypes coreTypes, ClassHierarchy hierarchy, this.strongMode)
       : super(coreTypes, hierarchy);
 
-  @override
-  final bool strongMode;
-
   /// Modify the given [constraint]'s lower bound to include [lower].
   void addLowerBound(TypeConstraint constraint, DartType lower) {
     constraint.lower = getLeastUpperBound(constraint.lower, lower);
@@ -226,13 +226,6 @@ class TypeSchemaEnvironment extends TypeEnvironment {
       List<DartType> actualTypes,
       DartType returnContextType,
       List<DartType> inferredTypes) {
-    if (returnContextType is DynamicType) {
-      // Analyzer treats a type context of `dynamic` as equivalent to an empty
-      // context.  TODO(paulberry): this is not spec'ed anywhere; do we still
-      // want to do this?
-      returnContextType = null;
-    }
-
     if (typeParametersToInfer.isEmpty) {
       return;
     }
@@ -243,7 +236,7 @@ class TypeSchemaEnvironment extends TypeEnvironment {
     // are implied by this.
     var gatherer = new TypeConstraintGatherer(this, typeParametersToInfer);
 
-    if (returnContextType != null) {
+    if (!isEmptyContext(returnContextType)) {
       gatherer.trySubtypeMatch(declaredReturnType, returnContextType);
     }
 
@@ -396,6 +389,16 @@ class TypeSchemaEnvironment extends TypeEnvironment {
     }
   }
 
+  bool isEmptyContext(DartType context) {
+    if (context is DynamicType) {
+      // Analyzer treats a type context of `dynamic` as equivalent to an empty
+      // context.  TODO(paulberry): this is not spec'ed anywhere; do we still
+      // want to do this?
+      return true;
+    }
+    return context == null;
+  }
+
   @override
   bool isTop(DartType t) {
     if (t is UnknownType) {