[wasm] [asm.js] Ignore unused function tables in AsmWasmBuilder.

Incremental parsing of asm.js means we can see function tables that
are unused in the AsmWasmBuilder before they've been initialized.

BUG=669899
R=aseemgarg@chromium.org

Review-Url: https://codereview.chromium.org/2546553002
Cr-Commit-Position: refs/heads/master@{#41403}

src/asmjs/asm-wasm-builder.cc

@@ -782,7 +782,10 @@ class AsmWasmBuilderImpl final : public AstVisitor<AsmWasmBuilderImpl> {
 
   void PopulateFunctionTable(VariableProxy* table, ArrayLiteral* funcs) {
     FunctionTableIndices* indices = LookupFunctionTable(table->var());
-    DCHECK_NOT_NULL(indices);
+    // Ignore unused function tables.
+    if (indices == nullptr) {
+      return;
+    }
     for (int i = 0; i < funcs->values()->length(); ++i) {
       VariableProxy* func = funcs->values()->at(i)->AsVariableProxy();
       DCHECK_NOT_NULL(func);

test/mjsunit/asm/regress-669899.js

+// 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.
+
+try {
+(function () {
+})();
+} catch(e) {; }
+  function __f_113() {
+  }
+(function () {
+function __f_89() {
+  "use asm";
+  function __f_63(__v_26, __v_28) {
+    __v_26 = __v_26|0;
+    __v_28 = __v_28|0;
+  }
+  function __f_21(table_id, fun_id, arg1, arg2) {
+    table_id = table_id|0;
+    fun_id = fun_id|0;
+    arg1 = arg1|0;
+    arg2 = arg2|0;
+  }
+  var __v_17 = [];
+}
+var module = __f_89();
+})();