Cross-compile from x64 to arm64 (#20)

lib/elinux_build_target.dart

@@ -12,6 +12,7 @@ import 'package:file/file.dart';
 import 'package:flutter_tools/src/artifacts.dart';
 import 'package:flutter_tools/src/base/common.dart';
 import 'package:flutter_tools/src/base/file_system.dart';
+import 'package:flutter_tools/src/base/os.dart';
 import 'package:flutter_tools/src/base/process.dart';
 import 'package:flutter_tools/src/build_info.dart';
 import 'package:flutter_tools/src/build_system/build_system.dart';
@@ -68,7 +69,7 @@ abstract class ELinuxAssetBundle extends Target {
         getBuildModeForName(environment.defines[kBuildMode]);
     final Directory outputDirectory = environment.outputDir
         .childDirectory('flutter_assets')
-          ..createSync(recursive: true);
+      ..createSync(recursive: true);
 
     // Only copy the prebuilt runtimes and kernel blob in debug mode.
     if (buildMode == BuildMode.debug) {
@@ -348,11 +349,24 @@ class NativeBundle {
 
     // Run the native build.
     final String cmakeBuildType = buildMode.isPrecompiled ? 'Release' : 'Debug';
+    final String targetArch = buildInfo.targetArch;
+    final String hostArch = _getCurrentHostPlatformArchName();
+    final String targetSysroot = buildInfo.targetSysroot;
+    final String systemIncludeDirectories = buildInfo.systemIncludeDirectories;
+    final bool needCrossBuild = targetArch != hostArch;
+    final bool needCrossBuildOptionsForArm64 = targetArch == 'arm64';
     RunResult result = await _processUtils.run(
       <String>[
         'cmake',
         '-DCMAKE_BUILD_TYPE=$cmakeBuildType',
         '-DFLUTTER_TARGET_BACKEND_TYPE=${buildInfo.targetBackendType}',
+        if (needCrossBuild) '-DFLUTTER_TARGET_PLATFORM_SYSROOT=$targetSysroot',
+        if (needCrossBuild && systemIncludeDirectories != null)
+          '-DFLUTTER_SYSTEM_INCLUDE_DIRECTORIES=$systemIncludeDirectories',
+        if (needCrossBuildOptionsForArm64)
+          '-DCMAKE_C_COMPILER_TARGET=aarch64-linux-gnu',
+        if (needCrossBuildOptionsForArm64)
+          '-DCMAKE_CXX_COMPILER_TARGET=aarch64-linux-gnu',
         eLinuxDir.path,
       ],
       workingDirectory: outputDir.path,
@@ -389,6 +403,11 @@ class NativeBundle {
   }
 }
 
+String _getCurrentHostPlatformArchName() {
+  final HostPlatform hostPlatform = getCurrentHostPlatform();
+  return getNameForHostPlatformArch(hostPlatform);
+}
+
 /// Converts [targetArch] to an arch name that corresponds to the `BUILD_ARCH`
 /// value used by the eLinux native builder.
 String getELinuxBuildArch(String targetArch) {