Prevent error on safely-handled requires using non-static arguments

Summary:
**Summary**
This is the workaround that cpojer suggested in #65 and should resolve the biggest pain point that developers are experiencing in that issue: using moment.js in React Native. Hopefully this at least serves as a starting point to reaching a decent solution for libraries using non-static `require()` statements.

**Test plan**
- Expected error when non-static-argument-based `require()` is nested more than one level deep within a `try` statement
- Expected no error when non-static-argument-based `require()` is nested directly within a `try` statement

```bash
ip-192-168-1-10:trueflipapp richard$ react-native bundle --entry-file index.js --platform ios --bundle-output /tmp/test.js --reset-cache
Scanning folders for symlinks in /repo/trueflip/sandbox/code/trueflipapp/node_modules (8ms)
Scanning folders for symlinks in /repo/trueflip/sandbox/code/trueflipapp/node_modules (8ms)
Loading dependency graph, done.
warning: the transform cache was reset.
bundle: start
bundle: finish
bundle: Writing bundle output to: /tmp/test.js
bundle: Done writing bundle output
```

Closes #69

Differential Revision: D6008567

Pulled By: cpojer

fbshipit-source-id: f9be328cf50dc47c7433ffeb5eb053b398c92122

packages/metro-bundler/src/JSTransformer/worker/__tests__/extract-dependencies-test.js

@@ -102,6 +102,22 @@ describe('Dependency extraction:', () => {
     );
   });
 
+  it('throws on calls to require with non-static arguments, nested deeper than one level inside of a try block', () => {
+    const code = "try { if (1) { require('foo/' + bar) } } catch (e) { }";
+
+    expect(() => extractDependencies(code)).toThrowError(
+      'require() must have a single string literal argument',
+    );
+  });
+
+  it('does not throw on calls to require with non-static arguments, nested directly inside of a try block', () => {
+    const code = "try { require('foo/' + bar) } catch (e) { }";
+
+    const {dependencies, dependencyOffsets} = extractDependencies(code);
+    expect(dependencies).toEqual([]);
+    expect(dependencyOffsets).toEqual([]);
+  });
+
   it('does not get confused by previous states', () => {
     // yes, this was a bug
     const code = 'require("a");/* a comment */ var a = /[a]/.test(\'a\');';

packages/metro-bundler/src/JSTransformer/worker/extract-dependencies.js

@@ -33,9 +33,13 @@ function extractDependencies(code: string) {
   const dependencies = new Set();
   const dependencyOffsets = [];
 
-  function pushDependency(nodeArgs) {
+  function pushDependency(nodeArgs, parentType) {
     const arg = nodeArgs[0];
     if (nodeArgs.length != 1 || arg.type !== 'StringLiteral') {
+      // Dynamic requires directly inside of a try statement are considered optional dependencies
+      if (parentType === 'TryStatement') {
+        return;
+      }
       throw new Error('require() must have a single string literal argument');
     }
     dependencyOffsets.push(arg.start);
@@ -46,8 +50,9 @@ function extractDependencies(code: string) {
     CallExpression(path) {
       const node = path.node;
       const callee = node.callee;
+      const parent = path.scope.parentBlock;
       if (callee.type === 'Identifier' && callee.name === 'require') {
-        pushDependency(node.arguments);
+        pushDependency(node.arguments, parent.type);
       }
       if (callee.type !== 'MemberExpression') {
         return;