Be more verbose when downloading artifacts

Previously, we would show 4 progress bars, which made it very unclear
what was going on since we'd just said "downloading engine". Now each
separate thing we download has its own announcement.

Eventually we should probably only download Dart, then do the rest of
this inside the flutter tool, as necessary.

bin/cache/update_engine.sh

@@ -11,7 +11,6 @@ ENGINE_STAMP_PATH="$FLUTTER_ROOT/bin/cache/engine.stamp"
 ENGINE_VERSION=`cat "$FLUTTER_ROOT/bin/cache/engine.version"`
 
 if [ ! -f "$ENGINE_STAMP_PATH" ] || [ "$ENGINE_VERSION" != `cat "$ENGINE_STAMP_PATH"` ]; then
-  echo "Downloading Flutter engine $ENGINE_VERSION..."
 
   BASE_URL="https://storage.googleapis.com/flutter_infra/flutter/$ENGINE_VERSION"
   PKG_PATH="$FLUTTER_ROOT/bin/cache/pkg"
@@ -19,9 +18,9 @@ if [ ! -f "$ENGINE_STAMP_PATH" ] || [ "$ENGINE_VERSION" != `cat "$ENGINE_STAMP_P
 
   # sky_engine Package
 
+  echo "Downloading Flutter engine $ENGINE_VERSION..."
   ENGINE_PKG_URL="$BASE_URL/sky_engine.zip"
   ENGINE_PKG_ZIP="$FLUTTER_ROOT/bin/cache/sky_engine.zip"
-
   curl --progress-bar -continue-at=- --location --output "$ENGINE_PKG_ZIP" "$ENGINE_PKG_URL"
   rm -rf -- "$PKG_PATH/sky_engine"
   unzip -o -q "$ENGINE_PKG_ZIP" -d "$PKG_PATH"
@@ -29,9 +28,9 @@ if [ ! -f "$ENGINE_STAMP_PATH" ] || [ "$ENGINE_VERSION" != `cat "$ENGINE_STAMP_P
 
   # sky_services Package
 
+  echo "  And corresponding services package..."
   SERVICES_PKG_URL="$BASE_URL/sky_services.zip"
   SERVICES_PKG_ZIP="$FLUTTER_ROOT/bin/cache/sky_services.zip"
-
   curl --progress-bar -continue-at=- --location --output "$SERVICES_PKG_ZIP" "$SERVICES_PKG_URL"
   rm -rf -- "$PKG_PATH/sky_services"
   unzip -o -q "$SERVICES_PKG_ZIP" -d "$PKG_PATH"
@@ -44,13 +43,13 @@ if [ ! -f "$ENGINE_STAMP_PATH" ] || [ "$ENGINE_VERSION" != `cat "$ENGINE_STAMP_P
 
   download_artifacts() {
     PLATFORM="$1"
-    PLATFORM_PATH="$ENGINE_ARTIFACT_PATH/$PLATFORM"
 
+    PLATFORM_PATH="$ENGINE_ARTIFACT_PATH/$PLATFORM"
     mkdir -p -- "$PLATFORM_PATH"
 
+    echo "  And corresponding toolchain for $PLATFORM..."
     ARTIFACTS_URL="$BASE_URL/$PLATFORM/artifacts.zip"
     ARTIFACTS_ZIP="$PLATFORM_PATH/artifacts.zip"
-
     curl --progress-bar -continue-at=- --location --output "$ARTIFACTS_ZIP" "$ARTIFACTS_URL"
     unzip -o -q "$ARTIFACTS_ZIP" -d "$PLATFORM_PATH"
     rm -f -- "$ARTIFACTS_ZIP"