Implement VariableMirror.isConst in the VM and ParameterMirror.isConst in dart2js.

tests/lib/lib.status

Index: tests/lib/lib.status
diff --git a/tests/lib/lib.status b/tests/lib/lib.status
index 16f2e2746acd14d228c7294e836b42c0823e58e8..17dfa2531f8c68e3229105d78287475b0dbf275c 100644
--- a/tests/lib/lib.status
+++ b/tests/lib/lib.status
@@ -80,6 +80,7 @@ mirrors/typedef_metadata_test: RuntimeError # Issue 12785
 mirrors/typevariable_mirror_metadata_test: CompileTimeError # Issue 10905
 mirrors/type_variable_owner_test/01: RuntimeError # Issue 12785
 mirrors/generic_type_mirror_test/01: RuntimeError # 6490
+mirrors/variable_is_const_test: RuntimeError # Issue 14671
 
 [ $runtime == safari ]
 mirrors/return_type_test: Pass, Timeout # Issue 12858