Decode main.js.map during snapshotting.

Pro:
time ./out/debug/deno tests/error_001.ts  3.0s -> 0.4s

Con:
time ./tool/build.py snapshot              33s -> 1m52s
out/debug/gen/snapshot_deno.bin            39M -> 121M

1 files changed, 6 insertions, 0 deletions

diff --git a/js/compiler.ts b/js/compiler.ts
index 941691f5c..a6b6e6970 100644
--- a/js/compiler.ts
+++ b/js/compiler.ts
@@ -388,6 +388,7 @@ export class DenoCompiler
     let lastModule: ModuleMetaData | undefined;
     sourceMaps.install({
       installPrepareStackTrace: true,
+
       getGeneratedContents: (fileName: string): string | RawSourceMap => {
         this._log("compiler.getGeneratedContents", fileName);
         if (fileName === "gen/bundle/main.js") {
@@ -419,6 +420,11 @@ export class DenoCompiler
         }
       }
     });
+    // Pre-compute source maps for main.js.map. This will happen at compile-time
+    // as long as Compiler is instanciated before the snapshot is created..
+    const consumer = sourceMaps.loadConsumer("gen/bundle/main.js");
+    assert(consumer != null);
+    consumer!.computeColumnSpans();
   }
 
   private constructor() {