Use native behavior for dead code elimination

tests/compiler/dart2js/js_spec_optimization_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.
+
+import 'dart:async';
+import 'package:async_helper/async_helper.dart';
+import 'package:expect/expect.dart';
+import 'compiler_helper.dart';
+
+const String TEST_1 = r"""
+  import 'dart:_foreign_helper';
+  main() {
+    // present: 'Moose'
+    JS('', 'Moose');
+
+    // absent: 'Phantom' - pure.
+    JS('returns: bool;effects:none;depends:none;throws:never', 'Phantom');
+
+    // present: 'Spider' - unused after constant folding 'is', but unpure.
+    print(JS('returns:bool;effects:none;depends:all', 'Spider') is bool);
+
+    // absent: 'Wasp' - unused after constant folding 'is', and unpure.
+    print(JS('returns:bool;effects:none;depends:all;throws:never', 'Wasp') is bool);

[floitsch, 2015/04/13 08:07:14]

nit: long line.

[sra1, 2015/04/13 23:59:05]

Done.

+
+    JS('', 'Array');   // absent: "Array"
+  }
+""";
+
+const String TEST_2 = r"""
+  import 'dart:_foreign_helper';
+  main() {
+    var w1 = JS('returns:int;depends:none;effects:none;throws:never',
+        'foo(#)', 1);
+    var w2 = JS('returns:int;depends:none;effects:none;throws:never',
+        'foo(#)', 2);
+
+    print([w2, w1]);
+
+    // present: '[foo(2), foo(1)]' - since 'foo' is pure, we expect to generate
+    // code out-of-order.
+  }
+""";
+
+const String TEST_3 = r"""
+  import 'dart:_foreign_helper';
+  main() {
+    var s = JS('String|Null', '"Hello"');
+    var s1 = JS('returns:String;depends:none;effects:none;throws:null(1)',
+        '#.toLowerCase()', s);
+    var s2 = JS('returns:String;depends:none;effects:none;throws:null(1)',
+        '#.toUpperCase()', s);
+    print(s2);
+
+    // absent: 'toLowerCase' - removed since s.toUpperCase() generates the same
+    // noSuchMethod.
+  }
+""";
+
+
+main() {
+  RegExp directivePattern = new RegExp(
+      //      \1                    \2        \3
+      r'''// *(present|absent): (?:"([^"]*)"|'([^'']*)')''',
+      multiLine: true);
+
+  Future check(String test) {
+    Uri uri = new Uri(scheme: 'dart', path: 'test');
+    var compiler = compilerFor(test, uri, expectedErrors: 0);
+    return compiler.runCompiler(uri).then((_) {
+      var element = findElement(compiler, 'main');
+      var backend = compiler.backend;
+      String generated = backend.assembleCode(element);
+
+      for (Match match in directivePattern.allMatches(test)) {
+        String directive = match.group(1);
+        String pattern = match.groups([2, 3]).where((s) => s != null).single;
+        if (directive == 'present') {
+          Expect.isTrue(generated.contains(pattern),
+              "Cannot find '$pattern' in:\n$generated");
+        } else {
+          assert(directive == 'absent');
+          Expect.isFalse(generated.contains(pattern),
+              "Must not find '$pattern' in:\n$generated");
+        }
+      }
+    });
+  }
+
+  asyncTest(() => Future.wait([
+    check(TEST_1),
+    check(TEST_2),
+    check(TEST_3),
+  ]));
+}