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 21 matching lines @@
 import 'diagnostics/spannable.dart' show
     NO_LOCATION_SPANNABLE,
     Spannable;
 import 'elements/elements.dart' as elements;
 import 'io/source_file.dart';
 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:'.

[sigurdm, 2015/10/19 07:26:26]

DBC: missing ')'

+///
+/// For example 'dart:html' has the environment variable 'dart.library.html' set
+/// to "true".
+const String dartLibraryEnvironmentPrefix = 'dart.library.';
+
 class Compiler extends leg.Compiler {
   api.CompilerInput provider;
   api.CompilerDiagnostics handler;
   final Uri libraryRoot;
   final Uri packageConfig;
   final Uri packageRoot;
   final api.PackagesDiscoveryProvider packagesDiscoveryProvider;
   Packages packages;
   List<String> options;
   Map<String, dynamic> environment;
@@ skipping 485 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) {
+    var result = environment[name];
+    if (result == null && !environment.containsKey(name) &&
+        name.startsWith(dartLibraryEnvironmentPrefix)) {
+      String libraryName = name.substring(dartLibraryEnvironmentPrefix.length);
+      LibraryInfo info = lookupLibraryInfo(libraryName);
+      if (info != null &&
+          allowedLibraryCategories.contains(info.category)) {
+        // 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;
+  }
 
   LibraryInfo lookupLibraryInfo(String libraryName) {
     return library_info.LIBRARIES[libraryName];
   }
 }