Fix analyzer's handling of import prefixes not followed by '.'.

tests/language/prefix_assignment_test.dart

+// Copyright (c) 2015, 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.
+
+// Validate that assignment to a prefix is handled consistently with the
+// following spec text from section 16.19 (Assignment):
+//     Evaluation of an assignment a of the form v = e proceeds as follows:
+//     Let d be the innermost declaration whose name is v or v=, if it exists.
+//     If d is the declaration of a local variable, ...
+//     If d is the declaration of a library variable, ...
+//     Otherwise, if d is the declaration of a static variable, ...
+//     Otherwise, if a ocurs inside a top level or static function (be it
+//   function, method, getter, or setter) or variable initializer, evaluation
+//   of a causes e to be evaluated, after which a NoSuchMethodError is thrown.
+//     Otherwise, the assignment is equivalent to the assignment this.v = e.
+//
+// Therefore, if p is an import prefix, evaluation of "p = ..." should be
+// equivalent to "this.p = ..." inside a method, and should produce a
+// NoSuchMethodError outside a method.
+
+import "package:expect/expect.dart";
+import "empty_library.dart" as p;
+
+class Base {
+  var p;
+}
+
+class Derived extends Base {
+  void f() {
+    p = 1; Expect.equals(1, this.p); /// 01: ok
+  }
+}
+
+bool gCalled = false;
+
+g() {
+  gCalled = true;
+  return 1;
+}
+
+noMethod(e) => e is NoSuchMethodError;
+
+main() {
+  new Derived().f();
+  Expect.throws(() { p = g(); }, noMethod); Expect.isTrue(gCalled); /// 02: static type warning
+}