Implement fromEnvironment on bool, int and String

sdk/lib/core/bool.dart

Index: sdk/lib/core/bool.dart
diff --git a/sdk/lib/core/bool.dart b/sdk/lib/core/bool.dart
index 6c7797fc4cb3be57f6fefaef74996aee1095a03e..bb74e56b255601c4d327a655f7021195b7977355 100644
--- a/sdk/lib/core/bool.dart
+++ b/sdk/lib/core/bool.dart
@@ -12,10 +12,12 @@ part of dart.core;
  * bool.
  */
 class bool {
-  factory bool._uninstantiable() {
-    throw new UnsupportedError(
-        "class bool cannot be instantiated");
-  }
+  /**
+   * Returns the boolean for the given environment variable [name] or
+   * [defaultValue] if [name] is not present.
+   */
+  external const factory bool.fromEnvironment(String name,
+                                              {bool defaultValue: false});
 
   /**
    * Returns [:"true":] if the receiver is [:true:], or [:"false":] if the