Improvements for checked mode: check when intializing fields, and when assigning to fields.

tests/language/language.status

Index: tests/language/language.status
===================================================================
--- tests/language/language.status	(revision 11874)
+++ tests/language/language.status	(working copy)
@@ -24,7 +24,6 @@
 gc_test: Fail # Issue 1487
 field_override_test/none: Fail  # Issue 742: field shadowing now allowed
 field_override_test/01: Fail    # Issue 742: field shadowing now allowed
-field_override_test/02: Fail    # Issue 742: field shadowing now allowed
 super_field_access_test: Fail    # Issue 742: field shadowing now allowed
 pseudo_kw_illegal_test/03: Fail  # Issue 356
 pseudo_kw_illegal_test/08: Fail  # Issue 356
@@ -76,6 +75,9 @@
 [ $compiler == none && $mode == debug ]
 gc_test: Skip  # Takes too long.
 
+[ $compiler == none && $unchecked ]
+field_override_test/02: Fail    # Issue 742: field shadowing now allowed
+
 [ $compiler == dartc ]
 
 get_set_syntax_test/none: Fail # does not accept getter/setter with no method body