[wasm][asm.js] Adding custom asm.js lexer.

test/unittests/BUILD.gn

 # Copyright 2016 The V8 project authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 import("../../gni/v8.gni")
 
 v8_executable("unittests") {
   testonly = true
 
   sources = [
     "../../testing/gmock-support.h",
     "../../testing/gtest-support.h",
     "api/access-check-unittest.cc",
     "api/exception-unittest.cc",
     "api/isolate-unittest.cc",
     "api/remote-object-unittest.cc",
     "api/v8-object-unittest.cc",
+    "asmjs/asm-scanner-unittest.cc",
     "base/atomic-utils-unittest.cc",
     "base/bits-unittest.cc",
     "base/cpu-unittest.cc",
     "base/division-by-constant-unittest.cc",
     "base/flags-unittest.cc",
     "base/functional-unittest.cc",
     "base/ieee754-unittest.cc",
     "base/iterator-unittest.cc",
     "base/logging-unittest.cc",
     "base/platform/condition-variable-unittest.cc",
@@ skipping 166 matching lines @@
     deps += [ "//third_party/icu" ]
   }
 
   if (is_win) {
     # This warning is benignly triggered by the U16 and U32 macros in
     # bytecode-utils.h.
     # C4309: 'static_cast': truncation of constant value
     cflags = [ "/wd4309" ]
   }
 }