Add boxing for modified variables

tests/corelib_2/corelib_2.status

 # Copyright (c) 2017, 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.
 
 # All static_tests have expected compile-time errors.
 [ $strong && $compiler != dart2analyzer && $compiler != dartdevc ]
 core_runtime_types_static_test: MissingCompileTimeError
 splay_tree_test/01: MissingCompileTimeError
 splay_tree_test/02: MissingCompileTimeError
 string_base_vm_static_test: MissingCompileTimeError
@@ skipping 477 matching lines @@
 const_list_set_range_test: Crash
 core_runtime_types_test: Crash
 data_uri_test: Crash
 dynamic_nosuchmethod_test: Crash # 'file:*/pkg/compiler/lib/src/ssa/locals_handler.dart': Failed assertion: line 425 pos 12: '!isStoredInClosureField(local)': Local local(NoSuchMethodError.toString#comma) is stored in a closure field.
 error_stack_trace1_test: Crash # 'file:*/pkg/compiler/lib/src/js_model/closure.dart': Failed assertion: line 197 pos 9: 'closure != null': Corresponding closure class not found for j:getter(Iterable.iterator). Closures found for (j:closure_call(Closure_fromTearOff_closure.call), j:closure_call(Maps_mapToString_closure.call), j:closure_call(unwrapException_closure.call))
 error_stack_trace2_test: Crash # Assertion failure: Unexpected constructor j:constructor(A.) in ConstructorDataImpl._getConstructorConstant
 expando_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
 growable_list_test: Crash
 hash_map2_test: Crash
-hash_map_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.
 hash_set_test/01: Crash
 hash_set_test/none: Crash
 hash_set_type_check_test: Crash # 'file:*/pkg/compiler/lib/src/js_model/closure.dart': Failed assertion: line 197 pos 9: 'closure != null': Corresponding closure class not found for j:getter(SetMixin.length). Closures found for (j:closure_call(Closure_fromTearOff_closure.call), j:closure_call(Maps_mapToString_closure.call), j:closure_call(main_closure.call), ..., j:closure_call(_LinkedCustomHashSet_closure.call), j:constructor(_LinkedCustomHashSet.))
 hidden_library2_test/01: Crash
 hidden_library2_test/none: Crash
 int_from_environment3_test/01: MissingCompileTimeError
 int_from_environment3_test/02: MissingCompileTimeError
 int_from_environment3_test/03: MissingCompileTimeError
 int_from_environment3_test/04: MissingCompileTimeError
 int_from_environment3_test/05: Crash
@@ skipping 43 matching lines @@
 list_concurrent_modify_test: Crash # Assertion failure: Missing scope info for j:method(Iterator.moveNext).
 list_contains_argument_order_test: Crash
 list_get_range_test: Crash
 list_insert_all_test: Crash
 list_insert_test: Crash
 list_map_test: Crash
 list_replace_range_test: Crash
 list_reversed_test: Crash
 list_set_all_test: Crash
 list_set_range_test: Crash
-list_sort_test: RuntimeError
 list_test/01: Crash
 list_test/none: Crash
 list_unmodifiable_test: Crash
 main_test: Crash
-map_contains_key_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.
 map_contains_value_test: Crash
 map_from_iterable_test: Crash
 map_from_iterables_test: Crash
 map_from_test: Crash
-map_index_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.
 map_keys2_test: Crash
 map_keys_test: Crash
-map_remove_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.
 map_test: Crash # Issue 27394
-map_to_string_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.
 map_values2_test: Crash
 map_values3_test: Crash
 map_values4_test: Crash
 map_values_test: Crash
 maps_test: Crash
 nan_infinity_test/01: RuntimeError
 null_nosuchmethod_test: Crash
 queue_first_test: Crash
 queue_iterator_test: Crash
 queue_last_test: Crash
@@ skipping 23 matching lines @@
 set_containsAll_test: Crash
 set_contains_test: Crash
 set_intersection_test: Crash
 set_iterator_test: Crash
 set_removeAll_test: Crash
 set_remove_test: Crash
 set_retainAll_test: Crash
 set_test: Crash
 set_to_string_test: Crash
 shuffle_test: Crash
-sort_test: RuntimeError
+expando_test: RuntimeError
 splay_tree_from_iterable_test: Crash
 splay_tree_from_iterables_test: Crash
 splay_tree_test/01: Crash
 splay_tree_test/02: Crash
 splay_tree_test/none: Crash
 stacktrace_fromstring_test: Crash
 string_buffer_test: RuntimeError
 string_codeunits_test: Crash
 string_from_environment3_test/01: MissingCompileTimeError
 string_from_environment3_test/02: MissingCompileTimeError
@@ skipping 170 matching lines @@
 list_concurrent_modify_test: Crash # NoSuchMethodError: The getter 'thisLocal' was called on null.
 list_contains_argument_order_test: Crash # NoSuchMethodError: The getter 'enclosingClass' was called on null.
 list_get_range_test: Crash # UnimplementedError: KernelClosedWorldMixin.getAppliedMixin
 list_insert_all_test: Crash # UnimplementedError: KernelClosedWorldMixin.getAppliedMixin
 list_insert_test: Crash # UnimplementedError: KernelClosedWorldMixin.getAppliedMixin
 list_map_test: Crash
 list_replace_range_test: Crash
 list_reversed_test: Crash
 list_set_all_test: Crash
 list_set_range_test: Crash
-list_sort_test: RuntimeError
 list_test/01: Crash
 list_test/none: Crash
 list_unmodifiable_test: Crash
 main_test: Crash
 map_contains_value_test: Crash
 map_from_iterable_test: Crash
 map_from_iterables_test: Crash
 map_from_test: Crash
 map_keys2_test: Crash
 map_keys_test: Crash
 map_test: Crash # Issue 27394
-map_to_string_test: RuntimeError
 map_values2_test: Crash
 map_values3_test: Crash
 map_values4_test: Crash
 map_values_test: Crash
 maps_test: Crash
 nan_infinity_test/01: RuntimeError
 null_nosuchmethod_test: Crash
 queue_first_test: Crash
 queue_iterator_test: Crash
 queue_last_test: Crash
@@ skipping 24 matching lines @@
 set_containsAll_test: Crash
 set_contains_test: Crash
 set_intersection_test: Crash
 set_iterator_test: Crash
 set_removeAll_test: Crash
 set_remove_test: Crash
 set_retainAll_test: Crash
 set_test: Crash
 set_to_string_test: Crash
 shuffle_test: Crash
-sort_test: RuntimeError
+regexp/ascii-regexp-subject_test: Crash # RangeError (index): Invalid value: Only valid value is 0: 1
 splay_tree_from_iterable_test: Crash
 splay_tree_from_iterables_test: Crash
 splay_tree_test/01: Crash
 splay_tree_test/02: Crash
 splay_tree_test/none: Crash
 stacktrace_fromstring_test: Crash
 string_base_vm_static_test: Crash
 string_buffer_test: RuntimeError
 string_codeunits_test: Crash
 string_from_environment3_test/01: MissingCompileTimeError
@@ skipping 88 matching lines @@
 
 [ $compiler == dart2analyzer && $strong  && !$checked ]
 iterable_fold_test/01: CompileTimeError
 
 [ $runtime == dart_precompiled && $minified ]
 apply_test: Skip  # Uses new Symbol via symbolMapToStringMap helper
 error_stack_trace1_test: Skip  # Expects unobfuscated stack trace
 
 [ $compiler == dart2js && $runtime != none && $dart2js_with_kernel ]
 list_concurrent_modify_test: Crash # Issue 30559