test(terser): sourcemap test

packages/terser/test/sourcemap/BUILD.bazel

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+load("@npm_bazel_jasmine//:index.from_src.bzl", "jasmine_node_test")
+load("@npm_bazel_terser//:index.from_src.bzl", "terser_minified")
+
+terser_minified(
+    name = "case1",
+    src = "src1.js",
+)
+
+jasmine_node_test(
+    name = "test",
+    srcs = [
+        "terser_spec.js",
+    ],
+    data = ["@npm//source-map"],
+    deps = [
+        ":case1",
+    ],
+)

packages/terser/test/sourcemap/README.md

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+The src1.js{,.map} files were generated from running `tsc` on src1.ts containing:
+
+```
+export class MyClass {
+    constructor(s: string) {
+        console.log(s);
+    }
+    field: string|undefined;
+}
+```

packages/terser/test/sourcemap/terser_spec.js

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+const fs = require('fs');
+const sm = require('source-map');
+const DIR = 'build_bazel_rules_nodejs/packages/terser/test/sourcemap';
+
+describe('terser sourcemap handling', () => {
+  it('should produce a sourcemap output', async () => {
+    const file = require.resolve(DIR + '/case1.js.map');
+    const rawSourceMap = JSON.parse(fs.readFileSync(file, 'utf-8'));
+    await sm.SourceMapConsumer.with(rawSourceMap, null, consumer => {
+      const pos = consumer.originalPositionFor({
+        // FIXME: this data doesn't seem right. src1.js doesn't have this on line 1
+        line: 1,
+        column: 17
+      });
+      expect(pos.name).toBe('MyClass');
+    });
+  });
+});