Add support for evaluating String.raw expressions (#5681)

* Add support for evaluating `String.raw` expressions

* Dedupe evaluation code for template literal quasis

* Check scoping in `String.raw` evaluation
  This addresses https://github.com/babel/babel/pull/5681#discussion_r114203497

* Ensure that `tag` is a MemberExpression in `String.raw` evaluation
  This addresses https://github.com/babel/babel/pull/5681#discussion_r114203555

* babel-traverse: Move evaluateQuasis() outside _evaluate()

packages/babel-traverse/src/path/evaluation.js

@@ -89,25 +89,20 @@ function _evaluate(path, state) {
   }
 
   if (path.isTemplateLiteral()) {
-    let str = "";
-
-    let i = 0;
-    const exprs = path.get("expressions");
-
-    for (const elem of (node.quasis: Array<Object>)) {
-      // not confident, evaluated an expression we don't like
-      if (!state.confident) break;
-
-      // add on cooked element
-      str += elem.value.cooked;
-
-      // add on interpolated expression if it's present
-      const expr = exprs[i++];
-      if (expr) str += String(evaluateCached(expr, state));
+    return evaluateQuasis(path, node.quasis, state);
   }
 
-    if (!state.confident) return;
-    return str;
+  if (path.isTaggedTemplateExpression() && path.get("tag").isMemberExpression()) {
+    const object = path.get("tag.object");
+    const { node: { name } } = object;
+    const property = path.get("tag.property");
+
+    if (
+      object.isIdentifier() && name === "String" && !path.scope.getBinding(name, true) &&
+      property.isIdentifier && property.node.name === "raw"
+    ) {
+      return evaluateQuasis(path, node.quasi.quasis, state, true);
+    }
   }
 
   if (path.isConditionalExpression()) {
@@ -349,6 +344,28 @@ function _evaluate(path, state) {
   deopt(path, state);
 }
 
+function evaluateQuasis (path, quasis: Array<Object>, state, raw = false) {
+  let str = "";
+
+  let i = 0;
+  const exprs = path.get("expressions");
+
+  for (const elem of quasis) {
+    // not confident, evaluated an expression we don't like
+    if (!state.confident) break;
+
+    // add on element
+    str += raw ? elem.value.raw : elem.value.cooked;
+
+    // add on interpolated expression if it's present
+    const expr = exprs[i++];
+    if (expr) str += String(evaluateCached(expr, state));
+  }
+
+  if (!state.confident) return;
+  return str;
+}
+
 /**
  * Walk the input `node` and statically evaluate it.
  *

packages/babel-traverse/test/evaluation.js

@@ -119,4 +119,16 @@ describe("evaluation", function () {
     assert.strictEqual(eval_undef.deopt.type, "VariableDeclarator");
     assert.strictEqual(eval_undef.deopt.parentPath.node.kind, "let");
   });
+
+  it("should work with String.raw", function () {
+    assert.strictEqual(
+      getPath("String.raw`\\d`").get("body")[0].evaluate().value,
+      "\\d"
+    );
+
+    assert.strictEqual(
+      getPath("`${String.raw`\\d`}`").get("body")[0].evaluate().value,
+      "\\d"
+    );
+  });
 });