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

tests/language/prefix_identifier_reference_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 the following spec text from section 16.32 (Identifier Reference):
+//     Evaluation of an identifier expression e of the form id proceeds as
+//   follows:
+//     Let d be the innermost declaration in the enclosing lexical scope whose
+//   name is id or id=.  If no such declaration exists in the lexical scope,
+//   d be the declaration of the inherited member named id if it exists.
+//     - If d is a prefix p, a compile-time error occurs unless the token
+//       immediately following d is '.'.
+
+import "package:expect/expect.dart";
+import "empty_library.dart" as p;
+
+void f(x) {}
+
+main() {
+  f(p);         /// 01: compile-time error
+  var x = p;    /// 02: compile-time error
+  var x = p[0]; /// 03: compile-time error
+  p[0] = null;  /// 04: compile-time error
+  p += 0;       /// 05: compile-time error
+}