Add support for 'dart.library.X' environment variables in dart2js.

pkg/compiler/lib/src/apiimpl.dart

 // Copyright (c) 2012, 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.
 
 library leg_apiimpl;
 
 import 'dart:async';
 import 'dart:convert';
 
 import 'package:package_config/packages.dart';
@@ skipping 15 matching lines @@
 import 'diagnostics/messages.dart' show
     Message;
 import 'elements/elements.dart' as elements;
 import 'io/source_file.dart';
 import 'platform_configuration.dart' as platform_configuration;
 import 'script.dart';
 
 const bool forceIncrementalSupport =
     const bool.fromEnvironment('DART2JS_EXPERIMENTAL_INCREMENTAL_SUPPORT');
 
+/// For every 'dart:' library, a corresponding environment variable is set
+/// to "true". The environment variable's name is the concatenation of
+/// this prefix and the name (without the 'dart:'.
+///
+/// For example 'dart:html' has the environment variable 'dart.library.html' set
+/// to "true".
+const String dartLibraryEnvironmentPrefix = 'dart.library.';
+
 /// Locations of the platform descriptor files relative to the library root.
 const String _clientPlatform = "lib/dart_client.platform";
 const String _serverPlatform = "lib/dart_server.platform";
 const String _sharedPlatform = "lib/dart_shared.platform";
 const String _dart2dartPlatform = "lib/dart2dart.platform";
 
 /// Implements the [Compiler] using a [api.CompilerInput] for supplying the
 /// sources.
 class CompilerImpl extends Compiler {
   api.CompilerInput provider;
@@ skipping 527 matching lines @@
   Future<Packages> callUserPackagesDiscovery(Uri uri) {
     try {
       return userPackagesDiscoveryTask.measure(
                  () => packagesDiscoveryProvider(uri));
     } catch (ex, s) {
       diagnoseCrashInUserCode('Uncaught exception in package discovery', ex, s);
       rethrow;
     }
   }
 
-  fromEnvironment(String name) => environment[name];
+  fromEnvironment(String name) {
+    assert(invariant(NO_LOCATION_SPANNABLE,
+        sdkLibraries != null, message: "setupSdk() has not been run"));
+    var result = environment[name];
+    if (result == null && !environment.containsKey(name) &&

[sigurdm, 2016/01/08 09:45:11]

The logic is a bit convoluted.

It might be easier to read
```
if (!environment.containsKey(name) &&
    name.startsWith(dartLibraryEnvironmentPrefix)) {
 [...]
} else {
  return environment[name];
}

[floitsch, 2016/01/08 15:51:01]

Refactored so that we have early returns.

+        name.startsWith(dartLibraryEnvironmentPrefix)) {
+      String libraryName = name.substring(dartLibraryEnvironmentPrefix.length);
+      if (sdkLibraries.containsKey(libraryName)) {
+        // Dart2js always "supports" importing 'dart:mirrors' but will abort
+        // the compilation at a later point if the backend doesn't support
+        // mirrors. In this case 'mirrors' should not be in the environment.
+        if (name == dartLibraryEnvironmentPrefix + 'mirrors') {
+          return backend.supportsReflection ? "true" : null;
+        }
+        return "true";
+      }
+    }
+    return result;
+  }
 
   Uri lookupLibraryUri(String libraryName) {
     assert(invariant(NO_LOCATION_SPANNABLE,
         sdkLibraries != null, message: "setupSdk() has not been run"));
     return sdkLibraries[libraryName];
   }
 
   Uri resolvePatchUri(String libraryName) {
     return backend.resolvePatchUri(libraryName, platformConfigUri);
   }
 }