Compute Compiler.proxyConstant on demand.

tests/compiler/dart2js/minimal_resolution_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.
+
+// Test that elements are not needlessly required by dart2js.
+
+import 'package:async_helper/async_helper.dart';
+import 'package:compiler/src/compiler.dart';
+import 'package:expect/expect.dart';
+import 'memory_compiler.dart';
+
+main() {
+  asyncTest(() async {
+    await analyze('main() {}');
+    await analyze('main() => proxy;', proxyConstant: true);
+  });
+}
+
+analyze(String code,
+        {bool proxyConstant: false}) async {
+  CompilationResult result = await runCompiler(
+      memorySourceFiles: {'main.dart': code},
+      options: ['--analyze-only']);
+  Expect.isTrue(result.isSuccess);
+  Compiler compiler = result.compiler;
+  Expect.equals(proxyConstant, compiler.proxyConstant != null);
+}