[wasm] Enforce that function bodies end with the \"end\" opcode.

test/mjsunit/wasm/wasm-module-builder.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.
 
 // Used for encoding f32 and double constants to bits.
 let __buffer = new ArrayBuffer(8);
 let byte_view = new Int8Array(__buffer);
 let f32_view = new Float32Array(__buffer);
 let f64_view = new Float64Array(__buffer);
 
@@ skipping 80 matching lines @@
     return this;
   }
 
   exportFunc() {
     this.exportAs(this.name);
     return this;
   }
 
   addBody(body) {
     this.body = body;
+    // Automatically add the end for the function block to the body.
+    body.push(kExprEnd);
     return this;
   }
 
   addLocals(locals) {
     this.locals = locals;
     return this;
   }
 
   end() {
     return this.module;
@@ skipping 435 matching lines @@
     }
     return buffer;
   }
 
   instantiate(ffi) {
     let module = new WebAssembly.Module(this.toBuffer());
     let instance = new WebAssembly.Instance(module, ffi);
     return instance;
   }
 }