v8: add type check to make failed calls visible

Calling v8.setFlagsFromString with e.g a function as a flag
argument gave no exception or warning that the function call will
fail. There is now an exception if the function gets called with
the wrong flag type (string is required) or that a flag is
expected.
Other APIs already provide exceptions if the argument has
not the expected type.

doc/api/v8.markdown

@@ -20,7 +20,7 @@ Returns an object with the following properties
 }
 ```
 
-## setFlagsFromString()
+## setFlagsFromString(string)
 
 Set additional V8 command line flags.  Use with care; changing settings
 after the VM has started may result in unpredictable behavior, including

src/node_v8.cc

@@ -60,6 +60,13 @@ void GetHeapStatistics(const FunctionCallbackInfo<Value>& args) {
 
 
 void SetFlagsFromString(const FunctionCallbackInfo<Value>& args) {
+  Environment* env = Environment::GetCurrent(args);
+
+  if (args.Length() < 1)
+    return env->ThrowTypeError("v8 flag is required");
+  if (!args[0]->IsString())
+    return env->ThrowTypeError("v8 flag must be a string");
+
   String::Utf8Value flags(args[0]);
   V8::SetFlagsFromString(*flags, flags.length());
 }

test/parallel/test-v8-flag-type-check.js

@@ -0,0 +1,6 @@
+var common = require('../common');
+var assert = require('assert');
+var v8 = require('v8');
+
+assert.throws(function() {v8.setFlagsFromString(1)}, TypeError);
+assert.throws(function() {v8.setFlagsFromString()}, TypeError);