Introduce collective --harmony flag.

test/mjsunit/harmony/block-let-crankshaft.js

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-// Flags: --harmony-block-scoping --allow-natives-syntax
+// Flags: --harmony-scoping --allow-natives-syntax
 
 // Test that temporal dead zone semantics for function and block scoped
 // ket bindings are handled by the optimizing compiler.
 
 function f(x, b) {
   let y = (b ? y : x) + 42;
   return y;
 }
 
 function g(x, b) {
@@ skipping 15 matching lines @@
   f(42, true);
 } catch (e) {
   assertInstanceof(e, ReferenceError);
 }
 
 try {
   g(42, true);
 } catch (e) {
   assertInstanceof(e, ReferenceError);
 }