- Implement a first cut for const String.env in the VM to allow

sdk/lib/core/string.dart

Index: sdk/lib/core/string.dart
===================================================================
--- sdk/lib/core/string.dart	(revision 28427)
+++ sdk/lib/core/string.dart	(working copy)
@@ -125,6 +125,13 @@
   }
 
   /**
+   * Returns the string for the given environment variable [name] or
+   * [defaultValue] if [name] is not present.

[Lasse Reichstein Nielsen, 2013/10/10 07:45:03]

We haven't specified what an "environment variable" is (or even an
"environment"). In a browser, it's not obvious that one exists, or if it does,
that its values are always strings.

That is, I think we should elaborate something like:

 An "environment" is a mapping from string "names" to string values
 that may be provided by the surrounding system. The environment
 may change between runs of the same program, but doesn't change 
 during a single execution.

maybe surrounding -> embedding, or is that too technical?

Is environment per-isolate? Can you change the environment when launching a new
isolate (preferably, yes)?

+   */
+  external const factory String.fromEnvironment(String name,
+                                                {String defaultValue: ''});
+
+  /**
    * Gets the character (as a single-code-unit [String]) at the given [index].
    *
    * The returned string represents exactly one UTF-16 code unit, which may be