Add boxing for modified variables

tests/compiler/dart2js_extra/dart2js_extra.status

 # Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 # for details. All rights reserved. Use of this source code is governed by a
 # BSD-style license that can be found in the LICENSE file.
 
 [ $compiler == dart2js ]
 class_test: Fail
 statements_test: Fail
 typed_locals_test: Fail
 no_such_method_test: Fail # Wrong Invocation.memberName.
 constant_javascript_semantics4_test: Fail, OK
@@ skipping 164 matching lines @@
 19191_test: Crash
 21666_test: Crash
 22868_test: Crash
 22917_test: Crash
 23404_test: RuntimeError
 23432_test: Crash
 23432b_test: Crash # type 'KMethod' is not a subtype of type 'MemberElement' of 'member' where
 23828_test: Crash
 assert_with_message_test: Crash
 async_stacktrace_test: Crash
-big_allocation_expression_test: Crash # 'file:*/pkg/compiler/lib/src/ssa/locals_handler.dart': Failed assertion: line 425 pos 12: '!isStoredInClosureField(local)': Local local(Maps.mapToString#first) is stored in a closure field.
 bound_closure_interceptor_methods_test: Crash
 bound_closure_interceptor_type_test: Crash
 break_test: Crash
 closure3_test: Crash
 closure4_test: RuntimeError
 closure6_test: Crash
 closure7_test: Crash
-closure_capture2_test: RuntimeError
 closure_capture3_test: Crash # Assertion failure: Cannot find value Instance of 'ThisLocal' in (local(Closure.nestedClosure#)) for j:closure_call(Closure_nestedClosure_closure.call).
 closure_capture4_test: RuntimeError
-closure_capture5_test: Crash # 'file:*/pkg/compiler/lib/src/ssa/locals_handler.dart': Failed assertion: line 425 pos 12: '!isStoredInClosureField(local)': Local local(closure4#g) is stored in a closure field.
-closure_capture_test: RuntimeError
 closure_type_reflection2_test: Crash
 closure_type_reflection_test: Crash
 code_motion_exception_test: Crash
 compile_time_constant4_test/01: MissingCompileTimeError
 compile_time_constant4_test/02: MissingCompileTimeError
 compile_time_constant4_test/03: MissingCompileTimeError
 compile_time_constant4_test/04: MissingCompileTimeError
 compile_time_constant4_test/05: MissingCompileTimeError
 compile_time_constant4_test/06: Crash
 conditional_send_test: Crash
@@ skipping 10 matching lines @@
 deferred/deferred_mirrors2_test: Crash
 deferred/deferred_overlapping_test: CompileTimeError
 deferred/interface_type_variable_test: CompileTimeError
 deferred/uninstantiated_type_variable_test: CompileTimeError
 deferred_custom_loader_test: CompileTimeError
 deferred_fail_and_retry_test: CompileTimeError
 deferred_fail_and_retry_worker_test: CompileTimeError
 deferred_inheritance_test: CompileTimeError
 deferred_split_test: CompileTimeError
 dummy_compiler_test: CompileTimeError
-first_class_types_hashcode_test: Crash # 'file:*/pkg/compiler/lib/src/ssa/locals_handler.dart': Failed assertion: line 425 pos 12: '!isStoredInClosureField(local)': Local local(Maps.mapToString#first) is stored in a closure field.
 for_in_test: Crash
 hash_code_test: Crash
-if_null2_test: Crash # 'file:*/pkg/compiler/lib/src/ssa/locals_handler.dart': Failed assertion: line 425 pos 12: '!isStoredInClosureField(local)': Local local(Maps.mapToString#first) is stored in a closure field.
-if_null3_test: Crash # 'file:*/pkg/compiler/lib/src/ssa/locals_handler.dart': Failed assertion: line 425 pos 12: '!isStoredInClosureField(local)': Local local(Maps.mapToString#first) is stored in a closure field.
+closure4_test: Crash # RangeError (index): Invalid value: Only valid value is 0: 1
 if_null_test: Crash
 inference_nsm_mirrors_test: Crash
 inference_super_set_call_test: RuntimeError
 interceptor_named_arguments_test: Crash
 invalid_annotation2_test/none: Crash
 is_check_instanceof_test: Crash
 js_dispatch_property_test: RuntimeError
 label_test/06: Crash
 lookup_map/dead_entry_single_nested_pairs_test: Crash
 lookup_map/dead_entry_test: Crash
@@ skipping 72 matching lines @@
 23828_test: Crash # NoSuchMethodError: Class 'KMethod' has no instance getter 'memberContext'.
 assert_with_message_test: Crash # NoSuchMethodError: The getter 'enclosingClass' was called on null.
 async_stacktrace_test: Crash # NoSuchMethodError: The getter 'memberIndex' was called on null.
 bound_closure_interceptor_methods_test: Crash # NoSuchMethodError: The getter 'enclosingClass' was called on null.
 bound_closure_interceptor_type_test: Crash # NoSuchMethodError: The getter 'classIndex' was called on null.
 break_test: Crash # RangeError (index): Invalid value: Valid value range is empty: 0
 closure3_test: Crash # NoSuchMethodError: Class 'KernelClosureClass' has no instance getter 'supertype'.
 closure4_test: RuntimeError
 closure6_test: Crash # NoSuchMethodError: Class 'KernelClosureClass' has no instance getter 'supertype'.
 closure7_test: Crash # NoSuchMethodError: Class 'KernelClosureClass' has no instance getter 'supertype'.
-closure_capture2_test: RuntimeError
 closure_capture3_test: Crash # Assertion failure: Cannot find value local(Closure_nestedClosure_closure.call#f) in (local(Closure.nestedClosure#)) for j:closure_call(Closure_nestedClosure_closure.call).
 closure_capture4_test: RuntimeError
-closure_capture5_test: RuntimeError
-closure_capture_test: RuntimeError
+closure4_test: Crash # RangeError (index): Invalid value: Only valid value is 0: 1
 closure_type_reflection2_test: Crash # UnimplementedError: KernelClosedWorldMixin.getAppliedMixin
 closure_type_reflection_test: Crash # UnimplementedError: KernelClosedWorldMixin.getAppliedMixin
 code_motion_exception_test: Crash
 compile_time_constant4_test/01: MissingCompileTimeError
 compile_time_constant4_test/02: MissingCompileTimeError
 compile_time_constant4_test/03: MissingCompileTimeError
 compile_time_constant4_test/04: MissingCompileTimeError
 compile_time_constant4_test/05: MissingCompileTimeError
 compile_time_constant4_test/06: Crash # NoSuchMethodError: The method 'fold' was called on null.
 conditional_send_test: Crash # NoSuchMethodError: The getter 'memberIndex' was called on null.
@@ skipping 85 matching lines @@
 timer_test: Crash # NoSuchMethodError: The getter 'memberIndex' was called on null.
 to_string_test: Crash
 truncation_errors_test: Crash # Assertion failure: Cannot find value local(main_closure.call#Infinity) in (local(main#)) for j:closure_call(main_closure.call).
 type_argument_factory_crash_test: Crash # Assertion failure: Cannot find value local(Maps_mapToString_closure.call#first) in (local(Maps_mapToString_closure.call#k), local(Maps_mapToString_closure.call#v), local(Maps.mapToString#)) for j:closure_call(Maps_mapToString_closure.call).
 type_argument_factory_nocrash_test: Crash # Assertion failure: Cannot find value local(Maps_mapToString_closure.call#first) in (local(Maps_mapToString_closure.call#k), local(Maps_mapToString_closure.call#v), local(Maps.mapToString#)) for j:closure_call(Maps_mapToString_closure.call).
 type_constant_switch_test/01: MissingCompileTimeError
 type_error_message_test: Crash # NoSuchMethodError: The getter 'enclosingClass' was called on null.
 unconditional_dartio_import_test: CompileTimeError
 useful_error_message_1_test: Crash # NoSuchMethodError: The getter 'enclosingClass' was called on null.