[wasm] Add support for a start function.

test/mjsunit/wasm/wasm-constants.js

 // Copyright 2015 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.
 
 // Flags: --expose-wasm
 
 function bytes() {
   var buffer = new ArrayBuffer(arguments.length);
   var view = new Uint8Array(buffer);
   for (var i = 0; i < arguments.length; i++) {
     var val = arguments[i];
     if ((typeof val) == "string") val = val.charCodeAt(0);
     view[i] = val | 0;
   }
   return buffer;
 }
 
 // Section declaration constants
 var kDeclMemory = 0x00;
 var kDeclSignatures = 0x01;
 var kDeclFunctions = 0x02;
 var kDeclGlobals = 0x03;
 var kDeclDataSegments = 0x04;
 var kDeclFunctionTable = 0x05;
+var kDeclStartFunction = 0x07;
 var kDeclEnd = 0x06;
 
 // Function declaration flags
 var kDeclFunctionName   = 0x01;
 var kDeclFunctionImport = 0x02;
 var kDeclFunctionLocals = 0x04;
 var kDeclFunctionExport = 0x08;
 
 // Local types
 var kAstStmt = 0;
@@ skipping 204 matching lines @@
       }
       threwException = false;
     } catch (e) {
       assertEquals("string", typeof e);
       assertEquals(kTrapMsgs[trap], e);
       // Success.
       return;
     }
     throw new MjsUnitAssertionError("Did not trap, expected: " + kTrapMsgs[trap]);
 }