Update documentation on .fromEnvironment. Make bool default be false.

sdk/lib/core/string.dart

Index: sdk/lib/core/string.dart
diff --git a/sdk/lib/core/string.dart b/sdk/lib/core/string.dart
index 8da434e7d520d7135dd0a571f7e031e34cbc3f28..799a2a96837e06652a54cb6cfb6b243d1ebea063 100644
--- a/sdk/lib/core/string.dart
+++ b/sdk/lib/core/string.dart
@@ -125,8 +125,22 @@ abstract class String implements Comparable<String>, Pattern {
   }
 
   /**
-   * Returns the string for the given environment variable [name] or
-   * [defaultValue] if [name] is not present.
+   * Returns the string value of the environment declaration [name].
+   *
+   * Environment declarations are provided by the surrounding system compiling
+   * or running the Dart program. Declarations map a string key to a string
+   * value.
+   *
+   * If [name] is not declared in the environment, the result is instead
+   * [defaultValue].
+   *
+   * Example of getting a value:
+   *
+   *     const String.fromEnvironment("defaultFloo", defaultValue: "no floo")
+   *
+   * Example of checking whether a declaration is there at all:
+   *
+   *     var isDeclared = const String.fromEnvironment("maybeDeclared") != null;
    */
   external const factory String.fromEnvironment(String name,
                                                 {String defaultValue});