Implement assumptions defined in the babel/rfcs#5 RFC

- `mutableTemplateObject` and `ignoreToPrimitiveHint` (#12408)
- `setClassMethods` (#12407)
- `setComputedProperties` (#12490)
- `ignoreFunctionLength` (#12491)
- `noDocumentAll` (#12481)
- `iterableIsArray` and `arrayLikeIsIterable` (#12489)
- `pureGetters` (#12504)
- `skipForOfIteratorClosing` (#12496)
- `objectRestNoSymbols`, `setSpreadProperties` and `pureGetters` (#12505)
- `noNewArrows` (#12613, #12793)
- `setPublicClassFields` and `privateFieldsAsProperties` (#12497)
- `constantReexports` and `enumerableModuleMeta` (#12618)
- `constantSuper`, `superIsCallableConstructor` and `noClassCalls` (#12726)

Co-authored-by: Justin Ridgewell <justin@ridgewell.name>
Co-authored-by: Huáng Jùnliàng <JLHwung@users.noreply.github.com>

packages/babel-plugin-transform-spread/test/fixtures/assumption-arrayLikeIsIterable/holes/exec.js

@@ -0,0 +1,6 @@
+var p2 = { 0: "a", 2: "c", length: 3 };
+
+var arr = [...p2, "d"];
+
+expect(arr).toEqual(["a", undefined, "c", "d"]);
+expect(1 in arr).toBe(true); // Not holey

packages/babel-plugin-transform-spread/test/fixtures/assumption-arrayLikeIsIterable/length-cropped/exec.js

@@ -0,0 +1,5 @@
+var p2 = { 0: "b", 1: "c", 2: "d", length: 2 };
+
+var arr = ["a", ...p2, "e"];
+
+expect(arr).toEqual(["a", "b", "c", "e"]);

packages/babel-plugin-transform-spread/test/fixtures/assumption-arrayLikeIsIterable/options.json

@@ -0,0 +1,9 @@
+{
+  "plugins": [
+    ["external-helpers", { "helperVersion": "7.100.0" }],
+    "transform-spread"
+  ],
+  "assumptions": {
+    "arrayLikeIsIterable": true
+  }
+}

packages/babel-plugin-transform-spread/test/fixtures/assumption-arrayLikeIsIterable/simple/exec.js

@@ -0,0 +1,5 @@
+var p2 = { 0: "b", 1: "c", 2: "d", length: 3 };
+
+var arr = ["a", ...p2, "e"];
+
+expect(arr).toEqual(["a", "b", "c", "d", "e"]);

packages/babel-plugin-transform-spread/test/fixtures/assumption-arrayLikeIsIterable/simple/input.js

@@ -0,0 +1 @@
+var arr = ["a", ...p2, "e"];

packages/babel-plugin-transform-spread/test/fixtures/assumption-arrayLikeIsIterable/simple/output.js

@@ -0,0 +1 @@
+var arr = ["a"].concat(babelHelpers.maybeArrayLike(babelHelpers.toConsumableArray, p2), ["e"]);