Add sourcemap support for asset-based random access bundles

Summary:This adds support for source maps that can be used for “random access modules” / “unbundles”

- source maps contain an extra custom field: `x_facebook_offsets`
- this field maps module IDs to line offsets
- the source map is built as if all files were concatenated

Decoding/symbolication works as follows:
- when decoding a stack trace, and a stack frame comes from a filename that contains only numbers and ends with `.js`, look up the additionally needed line offset in the offset map and add it to the original line of the stack frame.
- consume the source map as usual

Reviewed By: martinbigio

Differential Revision: D3072426

fb-gh-sync-id: 827e6dc13b1959f02903baafa7f9e4fc2e0d4bb9
shipit-source-id: 827e6dc13b1959f02903baafa7f9e4fc2e0d4bb9

local-cli/bundle/output/unbundle/util.js

@@ -0,0 +1,60 @@
+/**
+ * Copyright (c) 2016-present, Facebook, Inc.
+ * All rights reserved.
+ *
+ * This source code is licensed under the BSD-style license found in the
+ * LICENSE file in the root directory of this source tree. An additional grant
+ * of patent rights can be found in the PATENTS file in the same directory.
+ */
+'use strict';
+
+const newline = /\r\n?|\n|\u2028|\u2029/g;
+const countLines =
+  string => (string.match(newline) || []).length + 1; // fastest implementation
+
+function lineToLineSourceMap(source, filename) {
+  // The first line mapping in our package is the base64vlq code for zeros (A).
+  const firstLine = 'AAAA;';
+
+  // Most other lines in our mappings are all zeros (for module, column etc)
+  // except for the lineno mappinp: curLineno - prevLineno = 1; Which is C.
+  const line = 'AACA;';
+
+  return {
+    version: 3,
+    sources: [filename],
+    mappings: firstLine + Array(countLines(source)).join(line),
+  };
+}
+
+const wrapperEnd = wrappedCode => wrappedCode.indexOf('{') + 1;
+
+const Section = (line, column, map) => ({map, offset: {line, column}});
+
+function combineSourceMaps({modules, withCustomOffsets}) {
+  let offsets;
+  const sections = [];
+  const sourceMap = {
+    version: 3,
+    sections,
+  };
+
+  if (withCustomOffsets) {
+    offsets = sourceMap.x_facebook_offsets = [];
+  }
+
+  let line = 0;
+  modules.forEach(({code, id, map, name}) => {
+    const hasOffset = withCustomOffsets && id != null;
+    const column = hasOffset ? wrapperEnd(code) : 0;
+    sections.push(Section(line, column, map || lineToLineSourceMap(code, name)));
+    if (hasOffset) {
+      offsets[id] = line;
+    }
+    line += countLines(code);
+  });
+
+  return sourceMap;
+}
+
+module.exports = {countLines, lineToLineSourceMap, combineSourceMaps};