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

pkg/pkg.status

Index: pkg/pkg.status
===================================================================
--- pkg/pkg.status	(revision 11954)
+++ pkg/pkg.status	(working copy)
@@ -32,5 +32,31 @@
 unittest/test/instance_test: Skip
 
 [ $compiler == dart2js && $checked ]
-fixnum/test/int_32_test: Fail # Issue 3720.
-fixnum/test/int_64_test: Fail # Issue 3720.
+# The following tests fail on int checks.
+# Issue 3720
+fixnum/test/int_32_test: Fail
+fixnum/test/int_64_test: Fail
+args/test/args_test: Fail
+intl/test/bidi_format_test: Fail
+intl/test/bidi_utils_test: Fail
+intl/test/date_time_format_local_test: Fail
+intl/test/date_time_format_test: Fail
+intl/test/intl_message_test: Fail
+intl/test/intl_test: Fail
+intl/test/number_format_test: Fail
+logging/test/logging_test: Fail
+unittest/test/matchers_test: Fail
+unittest/test/mock_test: Fail
+unittest/test/unittest_test: Fail
+args/test/args_test: Fail
+intl/test/bidi_format_test: Fail
+intl/test/bidi_utils_test: Fail
+intl/test/date_time_format_test: Fail
+intl/test/intl_message_test: Fail
+intl/test/intl_test: Fail
+intl/test/number_format_test: Fail
+logging/test/logging_test: Fail
+unittest/test/matchers_test: Fail
+unittest/test/mock_test: Fail
+unittest/test/unittest_test: Fail
+yaml_test: Fail