Implement type inference for await expressions in front_end.

pkg/front_end/lib/src/fasta/kernel/kernel_shadow_ast.dart

 // Copyright (c) 2017, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 /// This file declares a "shadow hierarchy" of concrete classes which extend
 /// the kernel class hierarchy, adding methods and fields needed by the
 /// BodyBuilder.
 ///
 /// Instances of these classes may be created using the factory methods in
 /// `ast_factory.dart`.
@@ skipping 92 matching lines @@
 }
 
 /// Shadow object for [AwaitExpression].
 class KernelAwaitExpression extends AwaitExpression
     implements KernelExpression {
   KernelAwaitExpression(Expression operand) : super(operand);
 
   @override
   DartType _inferExpression(
       KernelTypeInferrer inferrer, DartType typeContext, bool typeNeeded) {
-    // TODO(scheglov): implement.
-    return typeNeeded ? const DynamicType() : null;
+    typeNeeded =
+        inferrer.listener.awaitExpressionEnter(this, typeContext) || typeNeeded;
+    if (!inferrer.typeSchemaEnvironment.isEmptyContext(typeContext)) {
+      typeContext = inferrer.wrapFutureOrType(typeContext);
+    }
+    var inferredType =
+        inferrer.inferExpression(operand, typeContext, typeNeeded);
+    inferredType = inferrer.typeSchemaEnvironment.flattenFutures(inferredType);
+    inferrer.listener.awaitExpressionExit(this, inferredType);
+    return inferredType;
   }
 }
 
 /// Concrete shadow object representing a statement block in kernel form.
 class KernelBlock extends Block implements KernelStatement {
   KernelBlock(List<Statement> statements) : super(statements);
 
   @override
   void _inferStatement(KernelTypeInferrer inferrer) {
     inferrer.listener.blockEnter(this);
@@ skipping 1402 matching lines @@
           closureContext.isAsync
               ? inferrer.coreTypes.streamClass
               : inferrer.coreTypes.iterableClass);
     }
     var inferredType = inferrer.inferExpression(
         expression, typeContext, closureContext != null);
     closureContext.handleYield(inferrer, isYieldStar, inferredType);
     inferrer.listener.yieldStatementExit(this);
   }
 }