(Babylon) Fall back to non-strict mode (#1587)

* (Babylon) Fall back to non-strict mode

This makes Prettier a little less opinionated about linting. For
example, the following can now be formatted:

```js
function f(a,a){return a}
```

whereas before it would cause an error:

    stdin: SyntaxError: Argument name clash in strict mode (1:13)
    > 1 | function f(a,a){return a}
        |              ^

This also allows octal numbers to be parsed,
and therefore fixes https://github.com/prettier/prettier/issues/228

If the code parses neither as strict nor as non-strict, the error from the
strict parse is thrown (as it was before this change).

---

I noticed this while trying out [eslump] with prettier:

    eslump | pbcopy; pbpaste | prettier

[eslump]: https://github.com/lydell/eslump

* Add missing test

* Use Object.assign() instead of mutating object

src/parser.js

@@ -60,7 +60,7 @@ function parseWithBabylon(text) {
   // Inline the require to avoid loading all the JS if we don't use it
   const babylon = require("babylon");
 
-  return babylon.parse(text, {
+  const babylonOptions = {
     sourceType: "module",
     allowImportExportEverywhere: false,
     allowReturnOutsideFunction: false,
@@ -77,7 +77,20 @@ function parseWithBabylon(text) {
       "functionSent",
       "dynamicImport"
     ]
-  });
+  };
+
+  try {
+    return babylon.parse(text, babylonOptions);
+  } catch (originalError) {
+    try {
+      return babylon.parse(
+        text,
+        Object.assign({}, babylonOptions, { strictMode: false })
+      );
+    } catch (nonStrictError) {
+      throw originalError;
+    }
+  }
 }
 
 function parseWithTypeScript(text) {

tests/non-strict/__snapshots__/jsfmt.spec.js.snap

@@ -0,0 +1,17 @@
+// Jest Snapshot v1, https://goo.gl/fbAQLP
+
+exports[`argument-name-clash.js 1`] = `
+function f(a,a){return a}
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+function f(a, a) {
+  return a;
+}
+
+`;
+
+exports[`octal-number.js 1`] = `
+0777
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+0777;
+
+`;

tests/non-strict/argument-name-clash.js

@@ -0,0 +1 @@
+function f(a,a){return a}

tests/non-strict/jsfmt.spec.js

@@ -0,0 +1 @@
+run_spec(__dirname, { parser: "babylon" });

tests/non-strict/octal-number.js

@@ -0,0 +1 @@
+0777