Restore functionality to run from .so file on Android (flutter#5278)

Flutter AOT builds can be done on Android using .so files (instead of
separate instruction/data snapshots) using the `--build-shared-library`
flag.

Running from .so files stopped working after the engine refactoring in
58e84c8, which this CL restores.

Issue flutter/flutter#17236

runtime/dart_snapshot.cc

@@ -21,6 +21,22 @@ const char* DartSnapshot::kIsolateDataSymbol = "kDartIsolateSnapshotData";
 const char* DartSnapshot::kIsolateInstructionsSymbol =
     "kDartIsolateSnapshotInstructions";
 
+#if defined(OS_ANDROID)
+// When assembling the .S file of the application, dart_bootstrap will prefix
+// symbols via an `_` to ensure Mac's `dlsym()` can find it (Mac ABI prefixes C
+// symbols with underscores).
+// But Linux ABI does not prefix C symbols with underscores, so we have to
+// explicitly look up the prefixed version.
+#define SYMBOL_PREFIX "_"
+#else
+#define SYMBOL_PREFIX ""
+#endif
+
+static const char* kVMDataSymbolSo = SYMBOL_PREFIX "kDartVmSnapshotData";
+static const char* kVMInstructionsSymbolSo = SYMBOL_PREFIX "kDartVmSnapshotInstructions";
+static const char* kIsolateDataSymbolSo = SYMBOL_PREFIX "kDartIsolateSnapshotData";
+static const char* kIsolateInstructionsSymbolSo = SYMBOL_PREFIX "kDartIsolateSnapshotInstructions";
+
 std::unique_ptr<DartSnapshotBuffer> ResolveVMData(const Settings& settings) {
   if (settings.vm_snapshot_data_path.size() > 0) {
     if (auto source = DartSnapshotBuffer::CreateWithContentsOfFile(
@@ -29,6 +45,14 @@ std::unique_ptr<DartSnapshotBuffer> ResolveVMData(const Settings& settings) {
     }
   }
 
+  if (settings.application_library_path.size() > 0) {
+    auto shared_library = fml::NativeLibrary::Create(settings.application_library_path.c_str());
+    if (auto source = DartSnapshotBuffer::CreateWithSymbolInLibrary(
+        shared_library, kVMDataSymbolSo)) {
+      return source;
+    }
+  }
+
   auto loaded_process = fml::NativeLibrary::CreateForCurrentProcess();
   return DartSnapshotBuffer::CreateWithSymbolInLibrary(
       loaded_process, DartSnapshot::kVMDataSymbol);
@@ -47,7 +71,7 @@ std::unique_ptr<DartSnapshotBuffer> ResolveVMInstructions(
     auto library =
         fml::NativeLibrary::Create(settings.application_library_path.c_str());
     if (auto source = DartSnapshotBuffer::CreateWithSymbolInLibrary(
-            library, DartSnapshot::kVMInstructionsSymbol)) {
+            library, kVMInstructionsSymbolSo)) {
       return source;
     }
   }
@@ -67,6 +91,15 @@ std::unique_ptr<DartSnapshotBuffer> ResolveIsolateData(
     }
   }
 
+  if (settings.application_library_path.size() > 0) {
+    auto library =
+        fml::NativeLibrary::Create(settings.application_library_path.c_str());
+    if (auto source = DartSnapshotBuffer::CreateWithSymbolInLibrary(
+            library, kIsolateDataSymbolSo)) {
+      return source;
+    }
+  }
+
   auto loaded_process = fml::NativeLibrary::CreateForCurrentProcess();
   return DartSnapshotBuffer::CreateWithSymbolInLibrary(
       loaded_process, DartSnapshot::kIsolateDataSymbol);
@@ -86,7 +119,7 @@ std::unique_ptr<DartSnapshotBuffer> ResolveIsolateInstructions(
     auto library =
         fml::NativeLibrary::Create(settings.application_library_path.c_str());
     if (auto source = DartSnapshotBuffer::CreateWithSymbolInLibrary(
-            library, DartSnapshot::kIsolateInstructionsSymbol)) {
+            library, kIsolateInstructionsSymbolSo)) {
       return source;
     }
   }

shell/common/switches.cc

@@ -170,6 +170,10 @@ blink::Settings SettingsFromCommandLine(const fxl::CommandLine& command_line) {
   command_line.GetOptionValue(FlagForSwitch(Switch::Packages),
                               &settings.packages_file_path);
 
+  std::string aot_shared_library_path;
+  command_line.GetOptionValue(FlagForSwitch(Switch::AotSharedLibraryPath),
+                              &aot_shared_library_path);
+
   std::string aot_snapshot_path;
   command_line.GetOptionValue(FlagForSwitch(Switch::AotSnapshotPath),
                               &aot_snapshot_path);
@@ -191,7 +195,9 @@ blink::Settings SettingsFromCommandLine(const fxl::CommandLine& command_line) {
       FlagForSwitch(Switch::AotIsolateSnapshotInstructions),
       &aot_isolate_snapshot_instr_filename);
 
-  if (aot_snapshot_path.size() > 0) {
+  if (aot_shared_library_path.size() > 0) {
+    settings.application_library_path = aot_shared_library_path;
+  } else if (aot_snapshot_path.size() > 0) {
     settings.vm_snapshot_data_path = fml::paths::JoinPaths(
         {aot_snapshot_path, aot_vm_snapshot_data_filename});
     settings.vm_snapshot_instr_path = fml::paths::JoinPaths(