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..bef70c649dcb712d84c22142016f5c3d140a4c62 100644
--- a/sdk/lib/core/bool.dart
+++ b/sdk/lib/core/bool.dart
@@ -12,10 +12,11 @@ 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.

[kasperl, 2013/10/30 13:32:49]

Shouldn't this actually have a defaultValue parameter? It is mentioned in the
comment and it does make sense even for booleans.

[Søren Gjesse, 2013/10/30 13:59:30]

In the latest design we removed the defaultValue for bool.fromEnvironment. It
seemed to have little value.

+   */
+  external const factory bool.fromEnvironment(String name);
 
   /**
    * Returns [:"true":] if the receiver is [:true:], or [:"false":] if the