Use a single file for app snapshots.

tools/testing/dart/compiler_configuration.dart

Index: tools/testing/dart/compiler_configuration.dart
diff --git a/tools/testing/dart/compiler_configuration.dart b/tools/testing/dart/compiler_configuration.dart
index d0e6f5f444ad02c68cd67c3c56f7e8c084c9c4ad..1daa2c154528a85c089b314945bfea40dcdda875 100644
--- a/tools/testing/dart/compiler_configuration.dart
+++ b/tools/testing/dart/compiler_configuration.dart
@@ -369,10 +369,12 @@ class PrecompilerCompilerConfiguration extends CompilerConfiguration {
       Map<String, String> environmentOverrides) {
     var exec = "$buildDir/dart_bootstrap";
     var args = new List();
-    args.add("--snapshot=$tempDir");
     args.add("--snapshot-kind=app-aot");
     if (useBlobs) {
+      args.add("--snapshot=$tempDir/out.aotsnapshot");
       args.add("--use-blobs");
+    } else {
+      args.add("--snapshot=$tempDir/out.S");
     }
     if (isAndroid && arch == 'arm') {
       args.add('--no-sim-use-hardfp');
@@ -431,7 +433,7 @@ class PrecompilerCompilerConfiguration extends CompilerConfiguration {
     args.addAll([
       '-o',
       '$tempDir/$libname',
-      '$tempDir/snapshot.S'
+      '$tempDir/out.S'
     ]);
 
     return commandBuilder.getCompilationCommand('assemble', tempDir, !useSdk,
@@ -447,7 +449,7 @@ class PrecompilerCompilerConfiguration extends CompilerConfiguration {
       List arguments,
       Map<String, String> environmentOverrides) {
     var exec = 'rm';
-    var args = ['$tempDir/snapshot.S'];
+    var args = ['$tempDir/out.S'];
 
     return commandBuilder.getCompilationCommand(
         'remove_assembly',
@@ -532,7 +534,7 @@ class Dart2AppSnapshotCompilerConfiguration extends CompilerConfiguration {
       Map<String, String> environmentOverrides) {
     var exec = "$buildDir/dart_bootstrap";
     var args = new List();
-    args.add("--snapshot=$tempDir");
+    args.add("--snapshot=$tempDir/out.jitsnapshot");
     args.add("--snapshot-kind=app-after-run");
     args.addAll(arguments);
 
@@ -592,7 +594,7 @@ class Dart2AppJitSnapshotCompilerConfiguration extends Dart2AppSnapshotCompilerC
       Map<String, String> environmentOverrides) {
     var exec = "$buildDir/dart";
     var args = new List();
-    args.add("--snapshot=$tempDir");
+    args.add("--snapshot=$tempDir/out.jitsnapshot");
     args.add("--snapshot-kind=app-jit-after-run");
     if (useBlobs) {
       args.add("--use-blobs");