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

tests/co19/co19-dart2js.status

Index: tests/co19/co19-dart2js.status
===================================================================
--- tests/co19/co19-dart2js.status	(revision 11874)
+++ tests/co19/co19-dart2js.status	(working copy)
@@ -812,10 +812,46 @@
 
 
 [ $compiler == dart2js && $checked ]
-# Leg does not support checked mode yet.
-*: Skip
 
+# The following tests fail on int checks.
+# Issue 3720
+Language/07_Classes/4_Instance_Variables_A02_t06: Fail
+Language/10_Expressions/16_Assignment/1_Compound_Assignment_A04_t04: Fail
+Language/11_Statements/05_For/2_For_in_A01_t01: Fail
+LibTest/core/Date/Date.fromString_A01_t01: Fail
+LibTest/core/Date/Date.now_A01_t01: Fail
+LibTest/core/Date/Date.now_A01_t02: Fail
+LibTest/core/Date/add_A01_t01: Fail
+LibTest/core/Date/compareTo_A01_t01: Fail
+LibTest/core/Date/compareTo_A01_t02: Fail
+LibTest/core/Date/day_A01_t01: Fail
+LibTest/core/Date/month_A01_t01: Fail
+LibTest/core/Date/subtract_A01_t01: Fail
+LibTest/core/Date/toString_A01_t01: Fail
+LibTest/core/Date/weekday_A01_t01: Fail
+LibTest/core/Duration/Duration_A01_t01: Fail
+LibTest/core/Duration/Duration_A02_t01: Fail
+LibTest/core/Duration/inDays_A01_t01: Fail
+LibTest/core/Duration/inHours_A01_t01: Fail
+LibTest/core/Duration/inMilliseconds_A01_t01: Fail
+LibTest/core/Duration/inMinutes_A01_t01: Fail
+LibTest/core/Duration/inSeconds_A01_t01: Fail
+LibTest/core/Stopwatch/elapsedInMs_A01_t01: Fail
+LibTest/core/Stopwatch/elapsedInUs_A01_t01: Fail
+LibTest/core/Stopwatch/elapsed_A01_t01: Fail
+LibTest/core/Stopwatch/elapsed_A01_t02: Fail
+LibTest/core/Stopwatch/frequency_A01_t01: Fail
+LibTest/core/Stopwatch/start_A01_t01: Fail
+LibTest/core/Stopwatch/start_A01_t02: Fail
+LibTest/core/Stopwatch/start_A01_t03: Fail
+LibTest/core/Stopwatch/stop_A01_t01: Fail
+LibTest/core/double/operator_remainder_A01_t02: Fail
+LibTest/core/double/operator_remainder_A01_t03: Fail
+LibTest/core/double/operator_remainder_A01_t04: Fail
+LibTest/core/double/operator_remainder_A01_t05: Fail
 
+LibTest/core/Queue/iterator_hasNext_A01_t01: Slow, Pass
+
 [ $compiler == dart2js && $browser ]
 *: Skip