[infra] Assembles the SDK using GN rather than create_sdk.py

runtime/bin/BUILD.gn

Index: runtime/bin/BUILD.gn
diff --git a/runtime/bin/BUILD.gn b/runtime/bin/BUILD.gn
index 19f0fd8c385898cb22fcac7eb745f01a7998af98..d446cb812c88a617cd7fe991abacee0f5fd75d51 100644
--- a/runtime/bin/BUILD.gn
+++ b/runtime/bin/BUILD.gn
@@ -321,7 +321,7 @@ executable("gen_snapshot") {
     ]
   }
 
-  if (defined(is_fuchsia) && is_fuchsia) {
+  if (is_fuchsia) {
     libs = [ "launchpad" ]
   }
 }
@@ -412,7 +412,7 @@ template("dart_io") {
       deps += [ "//third_party/boringssl" ]
     }
 
-    if (defined(is_fuchsia) && is_fuchsia) {
+    if (is_fuchsia) {
       libs = [ "launchpad" ]
     }
 
@@ -428,7 +428,7 @@ template("dart_io") {
                  "log.h",
                ] + extra_sources
 
-    if (is_linux || is_win || (defined(is_fuchsia) && is_fuchsia)) {
+    if (is_linux || is_win || is_fuchsia) {
       if (dart_use_fallback_root_certificates) {
         sources += [ "//third_party/root_certificates/root_certificates.cc" ]
       } else {
@@ -608,7 +608,7 @@ template("dart_executable") {
       ]
     }
 
-    if (defined(is_fuchsia) && is_fuchsia) {
+    if (is_fuchsia) {
       libs = [ "launchpad" ]
     }
   }
@@ -692,7 +692,7 @@ dart_executable("dart_bootstrap") {
   ]
 }
 
-if (defined(is_fuchsia) && is_fuchsia) {
+if (is_fuchsia) {
   hello_fuchsia_source = rebase_path("../tests/vm/dart/hello_fuchsia_test.dart")
 
   copy("hello_fuchsia") {
@@ -771,7 +771,7 @@ action("generate_snapshot_test_dat_file") {
 }
 
 executable("run_vm_tests") {
-  if (defined(is_fuchsia) && (is_fuchsia || is_fuchsia_host)) {
+  if (is_fuchsia || is_fuchsia_host) {
     testonly = true
   }