[DevTools] better breakpoints with source map

Some source map does not contain entry exactly at the end of the line,
so our frontend fails to find a mapping and ignores current line.

This CL tunes sourcemap to return last mapping in the line if there is
no mapping with offset greater then passed column.

R=dgozman@chromium.org

Bug: chromium:836440
Change-Id: I0da57992ca2c94101084aef2ed21c5b5ba6edc0a
Reviewed-on: https://chromium-review.googlesource.com/1028510
Commit-Queue: Aleksey Kozyatinskiy <kozyatinskiy@chromium.org>
Reviewed-by: Dmitry Gozman <dgozman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#555626}

third_party/WebKit/LayoutTests/http/tests/devtools/sources/debugger-breakpoints/breakpoint-with-sourcemap-dart-expected.txt

+Checks breakpoint in file with dart sourcemap
+breakpoint at 2
+breakpoint at 3
+

third_party/WebKit/LayoutTests/http/tests/devtools/sources/debugger-breakpoints/breakpoint-with-sourcemap-dart.js

+// Copyright 2018 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+(async function test() {
+  TestRunner.addResult('Checks breakpoint in file with dart sourcemap');
+  await TestRunner.loadModule('sources_test_runner');
+  await TestRunner.showPanel('sources');
+  await TestRunner.addScriptTag('resources/breakpoint.js');
+  let sourceFrame = await new Promise(
+      resolve =>
+          SourcesTestRunner.showScriptSource('breakpoint.dart', resolve));
+  SourcesTestRunner.toggleBreakpoint(sourceFrame, 2, false);
+  SourcesTestRunner.toggleBreakpoint(sourceFrame, 3, false);
+  await SourcesTestRunner.waitDebuggerPluginBreakpoints(sourceFrame);
+  await SourcesTestRunner.waitUntilDebuggerPluginLoaded(sourceFrame);
+  SourcesTestRunner.dumpDebuggerPluginBreakpoints(sourceFrame);
+  SourcesTestRunner.completeDebuggerTest();
+})();
\ No newline at end of file

third_party/WebKit/LayoutTests/http/tests/devtools/sources/debugger-breakpoints/resources/breakpoint.dart

+void _foo(
+    String a, List b) {
+  var val1 = a.codeUnits.length as int;
+  var val2 = a.runes.last;
+
+  var val3 = b.isNotEmpty
+    ? 42
+    : null;
+  print(val3);
+}
+
+void main() {
+  _foo("hello", []);
+}
+

third_party/WebKit/LayoutTests/http/tests/devtools/sources/debugger-breakpoints/resources/breakpoint.js

+define(['dart_sdk'], function(dart_sdk) {
+  'use strict';
+  const core = dart_sdk.core;
+  const dart = dart_sdk.dart;
+  const dartx = dart_sdk.dartx;
+  const _root = Object.create(null);
+  const breakpoint = Object.create(_root);
+  const $length = dartx.length;
+  const $codeUnits = dartx.codeUnits;
+  const $runes = dartx.runes;
+  const $isNotEmpty = dartx.isNotEmpty;
+  let StringAndListTovoid = () => (StringAndListTovoid = dart.constFn(dart.fnType(dart.void, [core.String, core.List])))();
+  let VoidTovoid = () => (VoidTovoid = dart.constFn(dart.fnType(dart.void, [])))();
+  breakpoint._foo = function(a, b) {
+    let val1 = a[$codeUnits][$length];
+    let val2 = a[$runes].last;
+    let val3 = dart.test(b[$isNotEmpty]) ? 42 : null;
+    core.print(val3);
+  };
+  dart.fn(breakpoint._foo, StringAndListTovoid());
+  breakpoint.main = function() {
+    breakpoint._foo("hello", []);
+  };
+  dart.fn(breakpoint.main, VoidTovoid());
+  dart.trackLibraries("breakpoint", {
+    "breakpoint.dart": breakpoint
+  }, null);
+  // Exports:
+  return {
+    breakpoint: breakpoint
+  };
+});
+
+//# sourceMappingURL=breakpoint.js.map

third_party/WebKit/LayoutTests/http/tests/devtools/sources/debugger-breakpoints/resources/breakpoint.js.map

+{"version":3,"sourceRoot":"","sources":["breakpoint.dart"],"names":[],"mappings":";;;;;;;;;;;;;6BACI,CAAQ,EAAE,CAAM;AAClB,QAAI,OAAO,CAAC,YAAU,SAAO;AAC7B,QAAI,OAAO,CAAC,QAAM,KAAK;AAEvB,QAAI,iBAAO,CAAC,aAAW,IACnB,KACA;AACJ,cAAK,CAAC,IAAI;EACZ;;;AAGE,mBAAI,CAAC,SAAS;EAChB","file":"breakpoint.js"}
\ No newline at end of file

third_party/blink/renderer/devtools/front_end/sdk/SourceMap.js

@@ -340,11 +340,11 @@ SDK.TextSourceMap = class {
     if (first >= mappings.length || mappings[first].sourceLineNumber !== lineNumber)
       return null;
     const columnMappings = mappings.slice(first, last);
+    if (!columnMappings.length)
+      return null;
     const index =
         columnMappings.lowerBound(columnNumber, (columnNumber, mapping) => columnNumber - mapping.sourceColumnNumber);
-    if (index >= columnMappings.length)
-      return null;
-    return columnMappings[index];
+    return index >= columnMappings.length ? columnMappings[columnMappings.length - 1] : columnMappings[index];
 
     /**
      * @param {number} lineNumber