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

pkg/compiler/lib/src/apiimpl.dart

Index: pkg/compiler/lib/src/apiimpl.dart
diff --git a/pkg/compiler/lib/src/apiimpl.dart b/pkg/compiler/lib/src/apiimpl.dart
index 8f81d151dcce9e238914c8b690bb1013e1e5b3d2..75280af1dc4fcccf11a203cd5297fd151f9db1c6 100644
--- a/pkg/compiler/lib/src/apiimpl.dart
+++ b/pkg/compiler/lib/src/apiimpl.dart
@@ -33,6 +33,14 @@ 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";
@@ -580,7 +588,25 @@ class CompilerImpl extends Compiler {
     }
   }
 
-  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,