Hoist current file name for transform-react-jsx-source

For better tooling support it's important to include absolute file name for JSX
elements. However, having them inline will inflate resulting file size, so we
move the file name to a constant declared at the beginning of the file and
reference it from `__source` attribute.

packages/babel-plugin-transform-react-jsx-source/src/index.js

@@ -10,30 +10,39 @@
  *
  * becomes:
  *
- * <sometag __source={{fileName: 'this/file.js', lineNumber: 10}}/>
+ * var __jsxFileName = 'this/file.js';
+ * <sometag __source={{fileName: __jsxFileName, lineNumber: 10}}/>
  */
 
 
-import path from "path";
-
 const TRACE_ID = "__source";
+const FILE_NAME_VAR = "__jsxFileName";
 
 export default function ({ types: t }) {
-  function makeTrace(fileName, lineNumber) {
+  function makeTrace(lineNumber) {
-    const fileNameLiteral = fileName != null ? t.stringLiteral(fileName) : t.nullLiteral();
     const fileLineLiteral = lineNumber != null ? t.numericLiteral(lineNumber) : t.nullLiteral();
-    const fileNameProperty = t.objectProperty(t.identifier("fileName"), fileNameLiteral);
+    const fileNameProperty = t.objectProperty(t.identifier("fileName"), t.identifier(FILE_NAME_VAR));
     const lineNumberProperty = t.objectProperty(t.identifier("lineNumber"), fileLineLiteral);
     return t.objectExpression([fileNameProperty, lineNumberProperty]);
   }
 
+  function makeFileNameConst(fileName) {
+    const declaration = t.variableDeclarator(t.identifier(FILE_NAME_VAR), t.stringLiteral(fileName));
+    return t.variableDeclaration("var", [declaration]);
+  }
+
   let visitor = {
-    JSXOpeningElement(node, state) {
+    Program(node, state) {
-      const id = t.jSXIdentifier(TRACE_ID);
+      const fileName = state.file.log.filename !== "unknown"
-      const fileName = state.file.log.filename !== "unknown" 
+        ? state.file.log.filename
-        ? path.relative(__dirname, state.file.log.filename) 
         : null;
-      const trace = makeTrace(fileName, node.container.openingElement.loc.start.line);
+
+      node.container.program.body.unshift(makeFileNameConst(fileName));
+    },
+
+    JSXOpeningElement(node) {
+      const id = t.jSXIdentifier(TRACE_ID);
+      const trace = makeTrace(node.container.openingElement.loc.start.line);
 
       node.container.openingElement.attributes.push(t.jSXAttribute(id, t.jSXExpressionContainer(trace)));
     }

packages/babel-plugin-transform-react-jsx-source/test/fixtures/react-source/basic-sample/expected.js

@@ -1,4 +1,5 @@
+var __jsxFileName = "/I/am/not/sure/how/to/get/path/to/test/fixtures/react-source/basic-sample/actual.js";
 var x = <sometag __source={{
-  fileName: "../test/fixtures/react-source/basic-sample/actual.js",
+  fileName: __jsxFileName,
   lineNumber: 1
-}} />;
+}} />;