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

sdk/lib/_internal/lib/core_patch.dart

Index: sdk/lib/_internal/lib/core_patch.dart
diff --git a/sdk/lib/_internal/lib/core_patch.dart b/sdk/lib/_internal/lib/core_patch.dart
index 8e5bec02c81cd96fb813f3328a2c64e52a228ba1..9e1c51f706c1467c15479b9bbb13f974ae99c9b2 100644
--- a/sdk/lib/_internal/lib/core_patch.dart
+++ b/sdk/lib/_internal/lib/core_patch.dart
@@ -232,7 +232,7 @@ patch class String {
 }
 
 patch class bool {
-  patch factory bool.fromEnvironment(String name, {bool defaultValue}) {
+  patch factory bool.fromEnvironment(String name, {bool defaultValue: false}) {
     throw new UnsupportedError(
         'bool.fromEnvironement can only be used as a const constructor');
   }