Add option to block-scoping to slow on throw code (#5236)

The let/const plugin can add closures where you don't expect them. This is undesirable in some perf-sensitive projects (ex: React). I added an option that throws whenever the plugin adds a function (as opposed to simply renaming variables when converting to var).

packages/babel-plugin-transform-es2015-block-scoping/test/fixtures/throwIfClosureRequired/for-const-closure/actual.js

@@ -0,0 +1,6 @@
+for (let i = 0; i < 5; i++) {
+  const l = i;
+  setTimeout(function() {
+    console.log(l);
+  }, 1);
+}

packages/babel-plugin-transform-es2015-block-scoping/test/fixtures/throwIfClosureRequired/for-const-closure/options.json

@@ -0,0 +1,3 @@
+{
+  "throws": "Compiling let/const in this block would add a closure (throwIfClosureRequired)."
+}

packages/babel-plugin-transform-es2015-block-scoping/test/fixtures/throwIfClosureRequired/function/actual.js

@@ -0,0 +1,3 @@
+function test() {
+  let foo = "bar";
+}

packages/babel-plugin-transform-es2015-block-scoping/test/fixtures/throwIfClosureRequired/function/expected.js

@@ -0,0 +1,3 @@
+function test() {
+  var foo = "bar";
+}

packages/babel-plugin-transform-es2015-block-scoping/test/fixtures/throwIfClosureRequired/options.json

@@ -0,0 +1,3 @@
+{
+  "plugins": [["transform-es2015-block-scoping", { "throwIfClosureRequired": true }], "syntax-jsx", "transform-react-jsx", "transform-es2015-block-scoped-functions", "transform-es2015-arrow-functions"]
+}

packages/babel-plugin-transform-es2015-block-scoping/test/fixtures/throwIfClosureRequired/superswitch/actual.js

@@ -0,0 +1,16 @@
+function foo() {
+  switch (2) {
+    case 0: {
+      if (true) {
+        return;
+      }
+
+      const stuff = new Map();
+      const data = 0;
+      stuff.forEach(() => {
+        const d = data;
+      });
+      break;
+    }
+  }
+}

packages/babel-plugin-transform-es2015-block-scoping/test/fixtures/throwIfClosureRequired/superswitch/options.json

@@ -0,0 +1,3 @@
+{
+  "throws": "Compiling let/const in this block would add a closure (throwIfClosureRequired)."
+}