Fix analyzer type checking of "await" expressions.

pkg/analyzer/lib/src/generated/static_type_analyzer.dart

Index: pkg/analyzer/lib/src/generated/static_type_analyzer.dart
diff --git a/pkg/analyzer/lib/src/generated/static_type_analyzer.dart b/pkg/analyzer/lib/src/generated/static_type_analyzer.dart
index 99a51ce11e396ed8769f890b92003b4000a95eaa..7b461df669cdc6f24e8fba46f306926a26b760f5 100644
--- a/pkg/analyzer/lib/src/generated/static_type_analyzer.dart
+++ b/pkg/analyzer/lib/src/generated/static_type_analyzer.dart
@@ -175,6 +175,34 @@ class StaticTypeAnalyzer extends SimpleAstVisitor<Object> {
   }
 
   /**
+   * The Dart Language Specification, 16.29 (Await Expressions):
+   *
+   *   Let flatten(T) = flatten(S) if T = Future<S>, and T otherwise.  The
+   *   static type of [the expression "await e"] is flatten(T) where T is the
+   *   static type of e.
+   */
+  @override
+  Object visitAwaitExpression(AwaitExpression node) {
+    DartType staticExpressionType = _getStaticType(node.expression);
+    if (staticExpressionType == null) {
+      // TODO(brianwilkerson) Determine whether this can still happen.
+      staticExpressionType = _dynamicType;
+    }
+    DartType staticType = flattenFutures(_typeProvider, staticExpressionType);
+    _recordStaticType(node, staticType);
+    DartType propagatedExpressionType = node.expression.propagatedType;
+    if (propagatedExpressionType != null) {
+      DartType propagatedType =
+          flattenFutures(_typeProvider, propagatedExpressionType);
+      if (propagatedType != null &&
+          propagatedType.isMoreSpecificThan(staticType)) {
+        _recordPropagatedType(node, propagatedType);
+      }
+    }
+    return null;
+  }
+
+  /**
    * The Dart Language Specification, 12.20: <blockquote>The static type of a logical boolean
    * expression is `bool`.</blockquote>
    *
@@ -1772,6 +1800,19 @@ class StaticTypeAnalyzer extends SimpleAstVisitor<Object> {
   }
 
   /**
+   * Implements the function "flatten" defined in the spec: "Let flatten(T) =
+   * flatten(S) if T = Future<S>, and T otherwise."
+   */
+  static DartType flattenFutures(TypeProvider typeProvider, DartType type) {
+    if (type is InterfaceType &&
+        type.element == typeProvider.futureType.element &&
+        type.typeArguments.length > 0) {
+      return flattenFutures(typeProvider, type.typeArguments[0]);
+    }
+    return type;
+  }
+
+  /**
    * Create a table mapping HTML tag names to the names of the classes (in 'dart:html') that
    * implement those tags.
    *