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/src/index.js

@@ -360,6 +360,12 @@ class BlockScoping {
   }
 
   wrapClosure() {
+    if (this.file.opts.throwIfClosureRequired) {
+      throw this.blockPath.buildCodeFrameError(
+        "Compiling let/const in this block would add a closure " +
+        "(throwIfClosureRequired)."
+      );
+    }
     const block = this.block;
 
     const outsideRefs = this.outsideLetReferences;