Add support to dart2js for option --enable-asserts.

tests/co19/co19-dart2js.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 ]
 Language/Classes/Constructors/Generative_Constructors/execution_t03: RuntimeError # https://github.com/dart-lang/sdk/issues/29596
 Language/Expressions/Function_Invocation/async_cleanup_t01: Skip # https://github.com/dart-lang/sdk/issues/28873
 Language/Expressions/Function_Invocation/async_cleanup_t03: Skip # https://github.com/dart-lang/sdk/issues/28873
 Language/Expressions/Function_Invocation/async_cleanup_t05: Skip # https://github.com/dart-lang/sdk/issues/28873
 Language/Expressions/Function_Invocation/async_cleanup_t06: Skip # https://github.com/dart-lang/sdk/issues/28873
@@ skipping 353 matching lines @@
 Language/Statements/Switch/type_t01: RuntimeError # Issue 16089
 Language/Statements/Return/runtime_type_t04: RuntimeError # Issue 26584
 Language/Types/Static_Types/malformed_type_t01: RuntimeError # Issue 21089
 Language/Types/Dynamic_Type_System/malbounded_type_error_t01: RuntimeError # Issue 21088
 Language/Types/Parameterized_Types/malbounded_t06: RuntimeError # Issue 21088
 LibTest/collection/DoubleLinkedQueue/DoubleLinkedQueue_class_A01_t01: Skip # Timesout. Please triage this failure.
 LibTest/core/Map/Map_class_A01_t04: Slow, Pass # Please triage this failure
 LibTest/core/Uri/Uri_A06_t03: Slow, Pass # Please triage this failure
 LibTest/math/Point/operator_mult_A02_t01: RuntimeError # Issue 1533
 
+[ $compiler == dart2js && ! $checked && $enable_asserts ]
+Language/Statements/Assert/execution_t01: SkipByDesign # Unspec'd feature.
+Language/Statements/Assert/execution_t02: SkipByDesign # Unspec'd feature.
+Language/Statements/Assert/execution_t03: SkipByDesign # Unspec'd feature.
+Language/Statements/Assert/execution_t04: SkipByDesign # Unspec'd feature.
+Language/Statements/Assert/execution_t05: SkipByDesign # Unspec'd feature.
+Language/Statements/Assert/execution_t06: SkipByDesign # Unspec'd feature.
+Language/Statements/Assert/execution_t11: SkipByDesign # Unspec'd feature.
+Language/Statements/Assert/production_mode_t01: SkipByDesign # Unspec'd feature.
+Language/Statements/Assert/type_t02: SkipByDesign # Unspec'd feature.
+Language/Statements/Assert/type_t03: SkipByDesign # Unspec'd feature.
+Language/Statements/Assert/type_t04: SkipByDesign # Unspec'd feature.
+Language/Statements/Assert/type_t05: SkipByDesign # Unspec'd feature.
+Language/Statements/Assert/type_t06: SkipByDesign # Unspec'd feature.
+
 [ $compiler == dart2js && $checked && $runtime != d8]
 LayoutTests/fast/canvas/webgl/framebuffer-object-attachment_t01: RuntimeError # Please triage this failure
 LayoutTests/fast/canvas/webgl/read-pixels-test_t01: RuntimeError # Please triage this failure
 LayoutTests/fast/inline/empty-inline-before-collapsed-space_t01: RuntimeError # Please triage this failure
 LayoutTests/fast/url/trivial_t01: RuntimeError # Please triage this failure
 LayoutTests/fast/url/trivial-segments_t01: RuntimeError # Please triage this failure
 LayoutTests/fast/writing-mode/auto-sizing-orthogonal-flows_t01: RuntimeError # Please triage this failure
 WebPlatformTest/html-templates/parsing-html-templates/clearing-the-stack-back-to-a-given-context/clearing-stack-back-to-a-table-body-context_t01: RuntimeError # Please triage this failure
 WebPlatformTest/html-templates/parsing-html-templates/clearing-the-stack-back-to-a-given-context/clearing-stack-back-to-a-table-context_t01: RuntimeError # Please triage this failure
 
@@ skipping 7986 matching lines @@
 WebPlatformTest/webstorage/storage_builtins_t01: RuntimeError # Please triage this failure
 WebPlatformTest/webstorage/storage_local_setitem_quotaexceedederr_t01: Skip # Times out. Please triage this failure
 
 [ $compiler == dart2js && $runtime == ie11 && $builder_tag == win8]
 LibTest/typed_data/ByteData/offsetInBytes_A01_t01: RuntimeError # Please triage this failure
 LayoutTests/fast/dom/XMLSerializer-attribute-entities_t01: RuntimeError # Please triage this failure
 LayoutTests/fast/dom/serialize-attribute_t01: RuntimeError # Please triage this failure
 
 [ $compiler == dart2js && $host_checked ]
 Language/Types/Function_Types/call_t01: Crash # Issue 28894