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 e515631bdb9163d8f06ebf1a796f860c2402b6ad..de068f1b5826d03d265166d2c2804d873fdd5182 100644
--- a/pkg/compiler/lib/src/apiimpl.dart
+++ b/pkg/compiler/lib/src/apiimpl.dart
@@ -39,6 +39,14 @@ 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:'.

[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;
@@ -544,8 +552,25 @@ class Compiler extends leg.Compiler {
     }
   }
 
-
-  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];